NEED HELP ASAP

Either get it from a reseller, or talk with the winner and see if they would agree to have it cancelled =(

Can't think of anything else, sorry

Buy a key from another site, humble bundle, green man gaming, use isthereanydeal.com to list the places you can buy the game and the price, probably won't be able to use the coupon, but there may be a voucher you can use.

Happen to me.
Twice.
FIrst time I gave the winner an item with an equivalent price to the game, so she can sell the item and buy the game.
Second time I made a support ticket and they deleted the giveaway. I rehosted it later with the right region restriction (but that is relevant to you).

EM: Shader Attack ? Get the key for it from tremorgames for 99 coins.. that's like 16 cents worth of tradeins or play some crappy flash games to get the coins.

It's not exactly region restricted, you can't send him the gift because the store price is different in your country than his/her country.

You can also try steamtrades.com to find a cheap copy

This happened to me recently too. I tried about 3 different games and got the same message each time. Gave up and got keys int he end. Great business model Steam!

I imagine that regions have slight price differences in most games. Does anybody know if there's an easy method of checking if games are giftable or not?

As XYU noted above, it's not an issue of publisher-based region restrictions (as would be covered by SteamDB), but with Valve's own sale policies for steam gifts. So long as the region in question has a 10% or higher currency value than that of your own region, you can't send to them. For example, the Euro has at times hovered around 15% higher exchange rate value than the US Dollar, meaning that sending to European nations from a US buyer wouldn't be allowed at those times.

Generally speaking, if you're buying directly from Steam, you'll want to make sure to region-restrict your giveaway to your own regional area (ie, Europe, Middle East, North America, CIS/RU).