I cant send a game as a gift

You can try to ask if the winner will agree to delete the giveaway or you can get the game on Greenmangaming and make sure it can be activated in their region.

The general rule of thumb is to never send steam gifts unless you know the price difference is under 10% or make it region-locked. If you know for sure the winner's region is supposed to be giftable and the winner is not from within your region-lock then send a ticket to support to ask for a reroll since it's not your fault if they aren't in your regions. If the winner IS from the regions specified then you'll have to try what I mentioned first.

To add to SnowyK''s comment:

Checking ITAD could give you a good baseline for current prices elsewhere, as well as historical prices. GMG seems to have the best pricing presently. You'll want to make sure to check region restrictions on the keys before purchasing them. Generally, the best bet is to find a store which sells ROW or WW keys.

Checking SteamDB allows you to determine the price difference for a given game (just make sure your local currency is selected in the selection box in the top right). You'll want to restrict any regions from your giveaway that utilize the currencies which have a price difference on SteamDB of +10% or higher (to be safe, round up if you're just a bit below 10%, since currency values do fluctuate). If that's too complicated, just restrict your giveaway to regions you feel comfortable with, such as ones which you know utilize the same currency as you.

As for your current giveaway, not much else to do but bring it up to the winner and staff, if you can't secure a replacement elsewhere, as there's really no way to directly work around Steam's internal restrictions.