I won a game a while back which was regifted by the user

Report the user. It's not allowed, so yeah.

I once forgot to activate TWO gifts from the same day (WHOOPS!) and was contacted by support (thankfully!) and told to activate them, so there IS a process in place... It's just a matter of time 'til it gets caught.

And yes, report them.

Oh goody, another one is in their trade topic.

you have 2 ga creators in your wins with unactivated wins, you can report both :3

The problem with auto checking things like this is that you can never prove that someone regifted. Sure, you can prove that they didn't activate a game they won, and you can know that they made a GA for that very same game, but you can't say for certain that they are regifting the copy they won. They may very well have bought another copy to give away and kept the one they won on their inventory (not likely, but still possible).

That's why you can report non-activation, but you can't report regifting.