Serious sam 2 HD not in my steamgifts' games list?

Run the game until it shows more than 0.0 hours played. Then re-sync.

run your serious sam second encounter and wait 10-15 minutes. Now close game and try resynch steamgifts account.

This is a common issue. It's a problem on Steam's end. It doesn't show on your profile's library, even with play time except when it does. It appears randomly in my experience. There's a few games like this and the moderators are aware of them. Don't worry about it.

SS2 does not have HD.