What can I do if the key does not work?

Contact Support

and by the way, you're not supposed to give away keys from Indie Gala

A number of people have reported lately that if you try to enter too many invalid keys (e.g. publicly posted ones that have already been used, ones with a missing letter that you have to guess, etc.) Steam will temporarily disable the ability to activate keys and report any the user tries as invalid. Ask him to give it a couple hours or a day or so and try it again, maybe?

Or maybe he's lying to get you to give him another key? It may be because i'm a cynical asshole, but that's how i would see it.

lol, this is a first for me

honestly, besides the fact that you broke the rules.

You should do nothing.

If it truely doesn't work, he'll contact SG support in the support tab.

i would say forget about it anyway your giveaway was against the rules and it might catch up to you, unless you send a support ticket telling what happened and since your giveaway was against the rules they might just cancel it out.