The Dimension Gates appear to be the means by which the Alien craft travel from their home world. Our craft cannot travel through these gates without being annihilated by an anti-matter implosion. We must disable a UFO and recover its control systems, propulsion systems and power sources for research. From: Apocalypse Ufopaedia
These strange portals allow UFOs to enter and exit Mega-Primus at will, but standard vehicles cannot. Dimension Gates are used to go to and from the Alien Dimension.
Their location throughout the city is random and they can change position at any time.
The very first research topic available of Quantum Physics is to understand this phenomenon. Ongoing research of alien exotics and UFOs will allow X-Com to eventually build a vehicle capable of traversing this gateway.
It is an invunerable object. To use it, a flying vehicle must use "Go To Map Point" (overhead view), targeting the gate.
If an X-Com craft is heading towards the alien's home, the view will change to the alien dimension at midnight. Once arrived, Mega-Primus becomes disabled and no actions can be performed 'back in the city' until any and all units have left the alien dimension via the gates. Once all craft have left and are in-transit, Mega-Primus immediately becomes available as normal but it takes a minimum of 24 hours for those crafts to return to base.
To use the time optimally, insert craft into the dimension gates just before midnight. Using your primary transport and main squad will leave Mega-Primus undefended. Do not send all your troops since you must be able to do missions in that 24 hour period after the main squad has left the aliens' homeworld.
To return from the alien dimension, use "Return To Base" button. The gates do not need to targeted via "Go To Map Point".
Image Oddity: Entering into the gate shows the best and final design of hybrid craft, an Annihilator. Returning to the city shows the first hybrid craft available to be built, a Dimension Probe. Showing the final craft is like a spoiler of what's to come.