The circuit is just a RS-232 to TTL adapter. Since the data port on the GP300 is bidirectional, a 7407 driver is used to control the data from PC to radio and from radio to PC. Since the battery must be removed to access the programming port, the circuit should also provide 7.5V to the walkie. That's why I included the shunt regulator and the 1N4148 diodes:

If you have any doubt about the circuit, this picture could help:

　　As I said, I also wanted something to connect the radio to the RIB easily. To prevent dangerous shorts, isolating lacker must be applied to the PCB, everywhere but over the contacts:

This is what it looks like while attached to the back of the GP300:

The final, wired product of your work should look similar to this: