So far, you seem to have considered only the AT89C5131 and the GPS.

Do you have an overall design for your complete system?

If not, you should start with that!

First, you need to define your Requirements; ie, what your system must do, and the constraints within which it must operate.

Once you have defined your Requirements, then you can start thinking about what will be needed to achieve those requirements; ie, a basic Architecture.

Once you have your Requirements and Architecture, then you can start thinking about what general types of components you could use to implement it.

Only then should you start getting into the detail of specific components, code, etc...