For some time I have been wishing this to happen, but since I have not seen it I will propose it here. If you make gems tradeable, it would give them real market value on TT. People playing for free would gain more options to buy from market. People buying them with real cash would still benefit since others need to produce the goods to buy gc to buy gems. Giving more shopping options to people not paying for this game would probably result the price of gems to be higher than now, and also result more real cash income to game runner.
Just for reference, Eve Online does this already, you can buy 30 day game time called Plex with your real live cash and then sell them for ingame cash called ISK. And from what I have heard, they are making a lot of money from it.
So I ask you all to think this for a while and point all good and bad things about it. Please do state your reasons when you comment this, thank you,
Kaakao