I agree with Russel - GPS receivers are commonly available that provide their output as a standard asychronous serial data stream.

Thus, interfacing this to an 8051 is just a standard serial interface problem.

Decoding the serial data stream (NMEA, or whatever) shouldn't require anything peculiar to the 8051.
Your GPS module will probably come with a description of the format - and possibly even sample code to decipher it. If not, look on the supplier's website, or try your favourite internet serach engine...