Einstein's theory only applies to conventional means of accelerating though. Engines will never work in this way. The most plausible way for a jump gate to work would be for it to work similarly to a wormhole, utilising the curvature of space to do a sort of a shortcut, much like what you could do if you curved a piece of paper then blown a hole through it with a pen. The relativity theory does not apply then, since the speed of light is never actually exceeded by the vessel. Considering how both gates and holes work similarly to how a wormhole would, I figure it's just different names for the same thing, except without the problems that modern physics points out, that are either partially or fully theoretical (like the need for exotic matter).
As a fun fact: Einstein's relativity theory says that the faster you travel relative to a non moving observer, the slower time goes for you relative to the observer (in this case the rest of humanity). So following this theory the moment you reach light speed relative to the rest of the universe, time stops for you. I'll leave you with the ensuing time paradox on your mind:P
