I have done some testing on the GPI case and the PSX standalone emulator.
the GPI case controller comes in hat mode by default and when using the PSX standalone emulator controller is a mess, but if you change the GPI case to axis and you remap the controller it works perfect.
Left, up right and down and bottom
only thing the I see is the games on my GPI case only show in haft of the screen with standalone emulator other than that emulation speed and sound is really good.
axis mode working good with all emulators I try, Atary2600, nes, Snes,megadrive ect....
Note Retropie psx standalone emulator controller also a mess if in hat mode, but works in axis mode
if anybody else could try it.