First of all, Just because the power supply can provde 2.5 amps, doesn't mean the hub will always pull 2.5 amps.
The Hub doesn't draw much power for itself, it needs 1/2 amp for each port on the hub, so 4 ports = 2 amps. Worst case, the hub is drawing 1/2 amp on its own, but that sounds a bit high, I'd guess 0.25 amps, but 1/2 amp max.
Your GPS won't draw more than 1/2 amp, USB regulations.
So add it up:
USB Hub 500mA (max)
GPS 500mA (max)
You can still plug at least one more device into the USB Hub.