This sounds like your fundamental problem.

GPRS relies upon IP (Internet Protocol) - so you need to do some reading-up on that.
TCP (Transmission Control Protocol) can be used on IP links, to give TCP/IP.
GPRS does not require TCP - you could use UDP.

Here are some links to get you started:
http://www.keil.com/tcpip/
http://www.computer-solutions.co.uk/chipdev/t...tcpwnw.htm
http://www.eix.co.uk/Ethernet/

A couple of free TCP/IP stacks are available - search here.

If you really want to write your own, you could try Jeremey Bentham's "TCP/IP Lean" book:
http://www.8052.com/forum/read.phtml?id=13386
(I think there's an updated edition now)

The examples are specifically for the PIC, but with 15 years' experience you should be able to adapt it to the 8051.