GTP-C vs GTP-U

I see ! Are there any GPRS simulators/emulator available , so that I can play around a little with GPRS and learn more about it. I have seen the name of GPRSim in a paper, but I couldn't find it on the net.

Also, I want to inject MS traffic logged at my handset into the Gn captured trace. More specifically I want to insert the MMS transaction flow observed at my side, in the GTP payload of the Gn interface. Are there any key points I need to adhere to, or is it just that I strip off some GTP headers and place them on top of my handset based traffic ?

Could you please relate any 3GPP specification relating MMS traffic to that of Gn traffic?