Can I create a GA with a game that I won which is in a bundle I buy later?

I think it depends on the time frame - I wouldn't give something away and at the same time try to win it (why?), but if you give it away thinking it's not for you and later find out more about it, or didn't like the genre and later you do, and you actually want it then, I think it's fine (I'd still feel weird about it myself, but we can't predict the future ...)

Eh, I just realized I've actually done this a bunch of times though, for wishlist giveaways XD But I specifically bought the copy for giveaways, usually unbundled games. When it's bundles I activate the games I want myself.

Well, I created GAs for games like Quantum Break or Dishonored 2 before I had them for myself and I would have loved to enter my own GAs :D

I even entred GAs for that games while my GA was running. (In the end I never won them so I had to buy me a second copy.... ^^)

It makes sense if you're minmaxing and don't need to play the game right now but would like to in the future. By giving it away early, you secure your CV (especially if it's non-bundled and might get bundled later), ratio and goodwill. If you then win it later, it'll be at a reduced value and 1:1. Of course, you run the risk of never winning it again and having to pay for another copy if you really want it in the future (still at reduced cost, presumably).
Also, for games that you don't plan on playing at all, you'd obviously give away, but still you might want to enter giveaways for them just to collect/idle/whatever.

Doing it sometimes with wishlisted games, because I think they're great and thus great gifts. Still trying to not buy it for myself then due to backlog reasons.