Won a game, but got the code for a different one.

No, you should not mark it as received. You can get a suspension if the game you won is not activated on your account and you won't be able to pass most SGTools protected giveaways. Instead contact the giveaway creator to tell him about it and ask for the right key. If he doesn't provide that mark as Not received after 7 days
Alternatively the creator can request from support to change his giveaway to the game he gave you if he gets an approval from you and the other winner

should i mark it as received?

No. Post in the giveaway that the key is for another game. The creator can send you the correct game or ask support to delete or change the giveaway with your permission. If not just mark it as not received after 7 days.

You can get blocked if Steamgifts checks that you don't have the game of the giveaway in your account, be 100% careful about it.

For example, I gave away Heaven Island VR MMO, turns out I had the key for Heaven Island VR (Non-mmo), the guy said that he wouldn't accept it. I told him I felt like shit for not being right, so I told him that I'd pick any game of 5€ of Steam (price of the original game) that he'd choose and gift it to him. He told me he wanted Batman Arkham City, so I gave it to him, but he still marked the giveaway as not received, becuase he didn't want to get blocked.

Mark it as not received or CONTACT the giveaway creator and tell him to give you the real game.

that's their fault...it's up to the creator to give you the original.

First: Do not mark it as received.

This looks like an honest mistake (same game title, different subtitle) and it does not look like either of them were given away free. If you are happy with the game you did receive, post in the giveaway that you are happy for the giveaway to be changed. Otherwise, inform the giveaway creator of the mistake and politely request the actual key, then mark as "not received" if you don't get it after 7 days.

Personally, I'd strongly recommend the former. As mentioned above, it looks like an honest mistake.

thanks to everybody for the advice, i will try to get in contact with the giveaway creator and see if the giveaway can be switched to the right one.