A Transit Gate is massive orbiting machine capable of forming a Einstein-Rosen bridge to pass matter or information instantly to another point in space; they also serve as functional space stations for interstellar travel and commerce. They are the primary (if not sole) method of traversing interstellar distances in the Solar Calendar setting.
|“||There are now thousands of gates throughout the volume of the galaxy, most still under TTA control. Floating in space orbiting the colony, each node is a city of hundreds of engineers, their families, and the infrastructure they need to live. At the center of that city is a machine capable of forming an Einstein-Rosen bridge to another point in space and passing matter or information through instantaneously.||”|
–Mobile Frame Zero: Rapid Attack rulebook, pg. 36
|“|| Transit gates have populations in the thousands sometimes. They're big.
There's a big, open, central dock area into which ships pop into existence, then move off for the next incoming/outgoing transit. They come out only approximately in the formation then entered in, so they give each other plenty of room on the way in. The docks are vulnerable to invasion! You don't really get to refuse an incoming transit, so unless they give some indication of malicious intent (which would mean shutting the transit gate down for what might turn into months, costing the transit corporation untold millions or billions of Wulongs), the TTMs on duty are really the defense. In your case, it would be whoever's got possession of the gate now. If there was an invasion here, you might find that the "attackers" are underprepared and wound up with their ship in a stupid position and can't get its frames here in time, making them effectively defenders; or that the defenders of the transit gate aren't up to military discipline and had a lag in who was on watch.
Then there's the city, probably in a ring around it, that has streets, shops, police (usually, really TTM) stations, and houses. If it's a ring (and the entire gate isn't in zero G), there are elevators going "up" to the docks. It might have parks and avenues, but they're probably scaled down.
Then, of course, is the immediate exterior of the gate. The "ground" of a ring-shaped station opens up into space, so small craft or frames can take a "down" elevator to launch themselves in any direction within the plane of the ring. Depending on the point you are in the orbit, that could be anywhere between a different orbit, deep space, or straight into the ground.
–Joshua A.C. Newman 
Lots of the parts that go into making and operating a transit gate have minimum gauges, but the really insurmountable challenge is support staff. It takes a lot of highly-trained engineers and physicists to set up and run one, at which point you're already building a small city around it, so why not build a gate big enough for cargo too?
|“||While it’s theoretically possible to send to any point in space, it is an extremely risky, and therefore expensive, process. Even the wealthy Sol system must come to consensus on the expenditure and time before beginning such an expedition. Because of the inherent imprecision of one-way transit, an expedition must start off fueled and equipped to make a journey through regular space from anywhere within the stellar system to the intended target. A small but noticeable number of such expeditions are never heard from again.||”|
–Mobile Frame Zero: Rapid Attack rulebook, pg. 36
There's no theoretical limit on transit gate jumps, but there are economic limits, unless you know of an easy way to convert the energy output of an entire solar system to power your jump. Square-cube law applies - a jump twice the distance will require much more than twice the power.
Gate to Gate TransitEdit
|“|| ...it is merely costly and not risky to transit to a receiving gate. With a transit gate reaching toward the beacon of another through the folds of space, the transmitter can contact the receiver and make the transit once the two have locked onto each other, making the transit more precise by orders of magnitude. This process of locking can take as little as a few days through clear space to as much as a few weeks when bypassing a black hole or other major hyperspatial phenomenon. Once the lock is made the transiting spacecraft arrive together in approximately the same formation as the one they left in, unraveling the connection between the two gates in the process.
Transit gates act as a hyperspatial beacon simply by virtue of their existence. When a transit gate shuts down, it can take weeks or months to bring it back up again, an unpredictable process reliant on the hyperspatial “weather” in their local area. While shut down, a gate cannot even communicate between the stars; it is genuinely isolated from the interstellar community. For this reason, a transit gate will not refuse an incoming transit under most circumstances - though traffic control can do so in emergencies, taking the entire colony offline in the process.
Turning off a transit gate is relatively easy but turning one on extremely time consuming (weeks to months to realign); in the meantime, no one can get out of the system and sending stuff in is extremely risky. 
|“|| Why is does the fuel fraction matter with an Einstein-Rosen bridge?"
"Because the technology they use takes a flat, non-negotiable amount of time to send anything other than zero mass information, and the resources it takes increase exponentially with the mass sent."
Transit gates make a wormhole across space, which is an infinite-but-brief perturbance in the fabric of spacetime(*Everyone in the transit gate city can feel when someone comes or goes. It's like living near an airport, only with your sense of balance instead of your hearing.). By putting it in freefall, it minimizes the effects on the nearby mass. Conversely, when you put it within a gravity well, it affects the gravity well and vice versa — Probably something you don't want.
I'd guess that, at a minimum, it would affect tides and cause earthquakes, not to mention throwing your precious cargo all over the place. Incoming ships don't come out in the exact formation in which they went in. Depending on unpredictable circumstances, they come out with attitudes and locations slightly different from those in which they went in. In space, there are rarely accidents because the transit gate is big enough to compensate for any fleet of normal density — say, a dozen ships or so. They more tightly they're packed, the more likely they'll crunch into each other. If you were to do this on a planet, some of the stuff would wind up underground and some would wind up way up in the air. I'm pretty sure that dropping an entire spacecraft a meter would cause some serious damage.
Oh, and unless the incoming space was completely evacuated of air (a volume that can hold a dozen ships in loose formation), all of the air will crash out of that space into the vacuum on the other side. Even if you had the gate at the other side on a planet with the atmosphere, the entire volume would suddenly equalize pressure.
So, the answer is, it presents huge, perhaps insurmountable engineering challenges.
A transit gate makes a hyperspatial bridge between two fourspace locations. In a vacuum, that's nice and simple: go in here, come out there (with a certain margin of error).
If you complicate things by having matter (such as fluids) or field effects (like gravity or magnetism) in those places, those forces interact for the instant that the wormhole is open. So if you have vacuum on one side and air on the other, the air will rush through.
So, if the volume you're connecting has mass on both ends — say, like, the inside of an asteroid on one side and a peach freighter on the other, I dunno. I guess I'm seeing nuclear fusion on both sides?
I'll give you my answer based on my aesthetic intentions: little mass-bumps in hyperspace are exactly what you don't want in your transit; they're cows on the train tracks. Furthermore, the presence of the mass makes it dicey enough that the sender is going to be unable to tell where the stuff they're sending is going with any safe precision; the greater the mass at the point of entry, the worse the signal; it's like the phone doesn't even ring on the other side. You could shove a ship through anyway, but you don't even know if it's going to wind up where the interfering mass is. It could hit smack on and fuse, it could show up a kilometer away in the transit gate's town square, sucking out the appropriate amount of air, it could show up a million kilometers away.
Well, it's like I have a sphere of point A at point B and vice-versa. Entering the sphere at one end is the same as exiting the sphere at the other. Except that it's very brief and the sphere is kinda wobbly, which means you pop out a little weird.
–Joshua A.C. Newman 
Transit gates must be built in free-fall but otherwise are found in a multitude of orbits depending on the needs of the colonies/systems they service.
Main Source: Mobile Frame Zero: Rapid Attack Rulebook
- ↑ http://www.mobileframehangar.com/viewtopic.php?f=9&t=2915#p23622
- ↑ http://www.mobileframehangar.com/viewtopic.php?f=9&t=4245#p35227
- ↑ http://www.mobileframehangar.com/viewtopic.php?f=9&t=4172#p34564
- ↑ http://www.mobileframehangar.com/viewtopic.php?f=9&t=2651
- ↑ http://www.mobileframehangar.com/viewtopic.php?f=9&t=744
- ↑ http://www.mobileframehangar.com/viewtopic.php?f=9&t=4305