The reason BB closed down the gem gift had nothing to do with scams. There are lots of opportunities to scam people in the game still, and it has never seemed like that big of a problem anyway. The real problem was that people could use the gem gift to circumvent the limit on gem pits on one account.
Account 1 has a bunch of gems and gem pits. Instead of using the gems to buy stuff, A1 sends the gems to A2. A2 buys a bunch of gem pits, then sends the gems back to A1 as they come in.
Since a gem pit doubles your gems, one player's number of gems could balloon completely out of proportion over the course of a couple of years. This would basically render gems worthless. Clearly, BB can't allow their real money currency to devalue to such an extent, so the gem gifts have to go. The only weird part is that they didn't realise this when they first decided to put in the gem gift.
The idea of a tradeable gem pit runs into the same issue, unfortunately.