So quickly, the Elite 4 types:
Bug, Ground, Fire and Psychic
Cynthia has a variety, we'll deal with her later.
My suggested Moveset:
- Ice Punch
- Drain Punch
- Swords Dance / Stone Edge (I don't care if you didn't mention it >:C)
- Night Slash / Stone Edge
Why you ask?
Ice Punch is the main weapon to beat Bertha. Her whole team, barr Whiscash is hit super effectively by it, and I doubt Whiscash can take a +2 Drain Punch from Gallade. While on the overall moveset, you lack something to hit bug, Ice Punch hits 2 of Aaron's Pokemon (Vespiqueen and Yanmega) for Super Effective damage :D. Ice Punch is also your best friend against Cynthia's Roserade, Garchomp and Togekiss.
Drain Punch is STAB and the recovery effect is very nice. Hits Flint's Houndoom, and most of his Fire types aren't that bulky anyway. It's also your best weapon against Cynthia's Milotic and Lucario. Drain Punch is also a good fallback to beat Aaron's Scizor, if you can get a +2 or something.
Swords Dance gives all of Gallade's moves the ability to basically OHKO your way through the Elite 4. Stone Edge is an interesting option if you can get your hands on it, as it rips through Flint's team as well as Aaron's.
Night Slash is probably the best move out of the remaining options you suggested. It's the best weapon for breaking through Lucian's team. Stone Edge can also be considered over it for the same reason, as Stone Edge will probably deal enough anyway, especially if you manage to get an SD up (Missing is many annoy tho).