Am I allowed to report a giveaway if I see something like that ?

He may have simply enjoyed it and bought it to give away. Is it on his account? And yes, you can report giveaways that you think are breaching the rules.

You could use the report button if you think something it's not right!

Use the report button, and if its a general misuse user, then put in a support ticket. State the problem clearly. If you can, also provide links to their "Win" pages, their "Regifting" page and/or the persons steam game library, showing game is not present etc. Anything that makes it easier for mod/admin to swiftly process the issue.

EDIT: But make sure you do your research. If they've won a game, and it is in their inventory and they are gifting a copy out, then thats not a violation. Winning Multiple of the same game (over a significant period of time) for example. Winning same game within a short period may simply be that they won a game because they had multiple entries.

For example, this user who giving out 2 copies of Faerie Solitaire right now, he's breaking rule ? He won 2 copies of Faerie Solitaire and he don't even have one copy set on his Steam Account.

And this moron gifted a guest pass. Not sure if this is allowed

ps: I apologize in advance for my bad engrish*