It isn't normal, but it is common. Cars are setup so accessories are turned off during cranking, but boats aren't. When the cranking load is placed on the battery, it can't hold whatever is the GPS's minimum voltage, so it shuts down. Could be a bad battery, a battery that doesn't have enough CCA for your outboard, bad wiring to the GPS, loose connections or bad wires from the battery to the starter, or a bad starter.