You you will use standard 9-pin DSUB connectors for your serial data, but you will not use standard RS232 signaling levels on these connectors? The first time someone tries to plug such a cable into your PC, something will break. Does that sound good to you?

Next thing. The GPS may jitter the position +/- 10 meters depending on what satellites it has managed to lock on to. How will that affect your navigation? If your robot is slow, the GPS jitter may actually be faster than the robot movement, so the robot runs forward but computes that the distance to the target is actually increasing.