Difference between GND and - on power supply?
You should connect the GND pin of your GPS module to the '-' pin of your power supply. The '+' and the '-' pins on the power supply are designed to be the outputs of the supply. The GND pin on the supply is completely isolated from '+' and '-', and is connected to Earth, literally. It exists as a safety precaution, in case the electronics inside the power supply's casing somehow comes in contact with the casing itself, it can be shorted to the Earth, and not shock the person using it.

Explained here to: https://www.eevblog.com/forum/beginners/bench-power-supply-confusion/