Well, in Star Wars there are certainly gravity engines... I remember this well, though I'm not 100% certain where it was stated first... I think it was in the George-Lucas-written novelization for the first film, actually.
The idea there was that ships would use a gravity-based drive whenever they were in the gravitational well of a large gravity source... a planet, a star, whatever. They'd basically push against the mass of that object for propulsion. They also had thrust-based stuff, but that wasn't sufficient to attain any kind of effective speeds. And of course, you have hyperspace, but that cannot be used in the vincinity of a large gravity well.
As time went on, and the "Expanded Universe" took over, the references to this sort of went by the wayside, but I remember when Star Wars FIRST came out, it was a common topic of conversation. And even though they pretty much stopped talking about it later on, the presence of gravitational control for flight, if anything, became MORE prevalent.
Just realize that an antigrav lift system would be useless without gravity to "reverse."
Now, in Star Trek... the talk about gravitation-based systems has been significantly more limited, and compartmentalized. You have artificial gravity for ships. You have antigravs for lifting objects. And you have tractor/pressor beams. However, it's pretty obvious (if never overtly stated) that shuttles use antigravity lift systems as well, and it's impossible for the Voyager to have landed, or for that matter to stay landed without collapsing the surface under those piddly little legs, unless gravitational control was actively in use the whole time. So it seems obvious that Trek uses gravity for propulsion as well... just that in Trek, you can only go "up" or "down" using that system, and you require thrusters for anything else, while in Star Wars, you can go pretty much every direction using just gravity.
Oh well... none of it is real anyway...
