Kuldeep, Impelementing HTTP is really very simple once you have impletented TCP/IP and PPP. Are you just looking for HTTP protocol. I have done this part and can help you easily on that.
I am also looking for a way to use the GPRS phone connected to a microcontroller to download a file. I have done the HTTP part but am stuck on GPRS and PPP connetion.
As keeping the source code as light as possible is the primary requirement. I studied the request-response of PC dial-up connection using a serial monitor.
Then i'm implementing the protocol according to the request-response sequence.
The book "PPP Design, Implementation and Debugging 2nd Edition" is very helpful.