The serial i won by a gift its invalid

Hmm, it could be the giveaway creator's fault, could've used the key already or the key is fake. Edit; oh also, maybe it's the owner typed it invalid or he bought an invalid key

ask the owner,
but maybe it's not steam, maybe it's just cd-key

Was it the Red Alert 3 giveaway? Those retail keys do not work for Steam.

I dont think there are steam redeemable Red Alert 3 cd keys (if that's the giveaway you are talking about).

Next time: Do not mark it as received before you activated it, if it isn't a valid Steam key it doesn't count. If you mark it as received and do not have the game on your account, it will be you who is in trouble for not activating a win (in 7 days you can change it though, or contact support, thats better and faster :p)

If you are a very lucky bastard and won another giveaway in this short timespan: ignore my advice and... congratulations (somehow)!

Not a steam key. Probably origin. you could activate there and steal the key to punish this guy, but you must mark as not received here.

In the end its just a giveaway. If he honestly didnt know it wouldnt work dont be a dick and just move along. If hes a dick about it go to support and mark as not received. sheesh you people are acting like you have this enormous entitlement when this guy offered a game out of his own good will and you are ready to scar his record permanently because he messed up... give the guy some slack...