Google Maps only uses the GPS when your phone's screen is on, but track recording apps will use the GPS continuously, even when the screen is off.
The USB standard is for 0.5A of current, however, that is not enough to power a smartphone with the screen and GPS both on. Most Android phones can work with 1.0A or even higher, but only through special circuitry in the battery pack (which is different from Apple-specific circuitry). The battery pack's specifications should tell you if it can do this. Some battery packs have 2 output ports, one with high-current Apple-specific circuitry and the other for Android devices. If you plug your cable in to the wrong port, you only get the default 0.5A.