The problem with powering the GPG robot with running motors (any version) with only USB power, is that the motors can cause dips or “brown-outs” on the 5v power supply rail. If the Raspberry Pi doesn’t have a well-regulated 5v power supply, it can crash and/or corrupt the SD card.
To prevent people from running the GPG without batteries, there is a software safety that prevents the GPG3 motors from running when there is inadequate power (batteries not connected). The GPG2 and GPG3 use very similar motors (with a much-improved encoder design on the GPG3).
To run motors without batteries, I recommend that you use a 9-12V DC regulated power supply (center positive), and plug it into the battery input barrel jack. Ideally it should be rated for 2A, but 1-1.5A might still work okay. I have extensively used 12V power supplies for developing with the GPG3.