Well my Raspberry Pi 3 is on it's way to me as we speak, and my old Gravis GamePad Pro USB is all I have lying around that isn't permanently needed elsewhere. And boy was it dusty! But it still works after plugging it into my Mac. Some solid gaming hardware IMO :-)

I used to use it in Classic Mac OS and OS X from '99 through to the mid-2000's, with classic console emulators, before I began buying old console titles on my modern consoles, instead. Only I've gotten tired of buying my classic console game library over and over again with every new modern console generation and purchase. So I've returned to emulation, this time by buying a Raspberry Pi 3 to use with my living room TV as a dedicated emulation box (perhaps with BOINC running when I'm not playing as well).

Now while my Macs can see this GamePad, I did often have to use a third party driver in order to be able to assign button functions (USB Overdrive in my case). Few apps did this on their own. All of which led me to my current doubts as to whether it would just work with RetroPie.

I'm very pleased to hear from you - obsidianspider - that there's no problem. Cheers for that! :-)