Most of the pins are actually free to use.
However no HAT is officially supported as we haven’t tested them. But you can totally try any, with Raspbian for Robots.
Here is a map of the GoPiGo3 to RPi interface that I created from the v3.2.0 schematic. Any pin with a blank GoPiGo3 function may be available BUT extreme caution is needed. The pins are low current 3v3, and static sensitive.
The GoPiGo3 seems designed to make things easy - built in level shifting, isolated power, LEDs, even multi-color LEDs, pre-built interface routines for a plethora of sensors and effectors from elementary school level to college-level.
“What are those pins for?” - for you. For your “mission to go where no [GoPiGo] has gone before.”