If you're US and doing direct gifting, you're probably covering most of the world, aside from probably a few european countries and the regions where the game itself isn't available.

As far as I know, you can't gift into any region where the game would be ≥10% more expensive than in the region you bought it from. So going by SteamDB, this would mean every region below (and including) Costa Rica in the list can't receive the gift.
Do double check, however, as I'm not 100% sure.

In the past I haven't been able to gift to Hungary from the US (that was several years ago, though). So I think any of Europe is at least in question. Again, please note this was years ago-- maybe things have changed. You can try gifting a $1 game to various friends on Steam and see whom it disallows...?

When making a giveaway like this you can look at steamdb for the pricing and exclude all regions you cannot gift, that way you do not need to reroll in case of someone winning in a region you cannot gift / run into a situation where that person does not agree to a reroll.

Thank you for all the comments regarding region and pricing

GTX 1080 still a beast GPU in my book. It's crazy how time flies. Very capable card but sadly will lose support at some point.