I may buy some games from this git gud promo. I already have Spelunky. So is it possible to buy Spelunky (as a gift) and new games together? If it's possible, how can I use Spelunky gift code?

To my knowledge, it's not possible. It'll turn the entire package into a gift so long as you own one item in it.
Edit: Just checked. I KNOW it's not possible.
What you'll have to do is buy the Spelunky game separately, then gift it to yourself by putting in your own email. You can either trade or give away the gift by giving the other person the code. Alternatively, if you already know the email of the person you want to gift it to, you can just gift it to them directly instead.

yup. buy them, extra gams will appear in the gift code under "orders and settings". ignore the mosquito. hes shit.

And don't forget:
4) Mark HunchBluntley as the solution.

2.a.)You may ask support to give you individual keys for each games.
2.b.)Wait for reply. And Proceed to Step 3.

You may want to be careful though. There was some mention in another thread that gifts may now use a different higher price. A defense for people buying from a cheap region and gifting it to others. You should check if it does for you.

If so, then much better to split into two purchases; one for gifts, one for own library. Thanks for the warning and have a +

Good point. :)

Thank you guys. +1 for your helpful posts. :)