Ships in ED move too fast for gravity assist to mean anything. It works for newtonian velocities. Velocities in ED except normal space flight are relativistic.
Because ships do not actually move through real space in supercruise, the term relativistic is inaccurate. In order to do that, we'd have to be traveling at those speeds in real space (not supercruise or witch space), but that would be incredibly difficult to achieve on a technical level, since time dilates at such speeds. Also, everything in front of you would blue shift and appear much, much closer than it actually was, and farther behind you, as well as red shifting. There are canon objects in the game moving at relativistic velocities in real space in the game, such as the Voyager probes (which I think have been located), and the ancient generation ships (which have not been located). But there is no ship in the Elite universe in 3302 that can realistically achieve that kind of speed in real space, and if a ship is in supercruise, it wouldn't be able to see these objects.