The Warp gate was a stellar phenomena that was used as a form of long distance travelling through a large portal in space. This allowed individuals to travel from one point in the galaxy to another almost instantenously.
The exact nature of the phenomena is not entirely known but may possibly be related to Space Bridge technology.
The Transformers cartoon
After the revival of Galvatron from the plasma pits on the planet Thrull, the band of Decepticons made their way back to Charr but detoured to Earth after their leader learnt of a Human space platform in the Sol system. Galvatron, Cyclonus, Scourge and the Sweeps traveled through the Warp Gate and arrived at Pluto.
Generation 2 comic
In the first issue of the Marvel Generation 2 comic, Sideswipe and other Autobot "Firestormers" attack a Cybertronian Empire occupational force by using Personal Warp Gates. No further information is given, but it would appear that it is a one-'bot, one-way, and perhaps one-use device.