modem microchip
Almost all GPRS modems have TCP/IP stack built in, so your job is quite easy. You will have to transfer raw data, as they are offered by the TCP/IP stack from Microchip directly to the modem, and do the same for the other direction. Of course, you will have to manage the ethernet socket, and also the GPRS connection/GSM modem (configuration commands, dial commands, supervise the status of the modem and of the connection), and also you will have to implement some kind of flow control and data buffering, since the speeds used in these connections are at a big difference.
/pisoiu