Why can't I give away my Gun Monkeys copy?

Because that game is so cool that the SG servers can't handle it.

It was free, almost everyone has it and people would abuse of free games to gain contributor value. So they get removed from the giveaway list.

You actually can give away your copy of Gun Monkeys. You simply drop the key in this forum posting and voila! Game given away.

As for making an SG giveaway with it, you can't do that with free or exploited games.

You sir, are an impostor >:/