Can I use Sudowoodo's "Watch and Learn" attack to copy an opponent's GX Attack used on their previous turn? If yes, does this count as my GX Attack for the game? And if I have already used my GX Attack for the game, does the "Watch and Learn" attack fail?

Yes, yes, and sort of. If you have already used your GX Attack you can still use Watch and Learn, but the effect of the copied GX Attack would fail.

Relevant cards