Good fight...I Guess it would depend on where about in the death star. If it's in a large room then Gibson would take it as his (and the other wanted assassins) are better from distance than Preston.
Close up...I think Preston's h2h is superior.
I guess it comes down to training vs natural ability.
I also think that if Wesley is supposed to be more skilled than Mr X and Mr X can generate enough speed while running to blow paper off tables in adjacent offices and jump a massive distance from one building to another then it's safe to say they are effectively superhuman.
So if I was to give it a "out of 10" ratio then i'd say Gibson takes it 6:4
All coming down to location within the death star (presumably they have to locate and hunt one another down so surprise may be a factor too).
You two are going to give Will Hatch a bloated head.
I think Gibson wins. His curving of bullets effectively counters Preston's gunkata. Preston will position himself according to Gibson's position expecting to avoid a straight flying bullet, when it won't be. That and Gibson's super-heigthened abilities/senses.