Having a hard time figuring out how to get a pico eight running on my RG405M with gamma OS. Anyone done this yet?

GammaOS is based on Lineage which in turn is based on Android - there's no Android version available for PICO-8, so I doubt you'd be able to use it under GammaOS. I also don't know of any emulators for Android, but maybe someone else does.

GammaOS does run Pico-8 in RetroArch. Simply download the Retro8 core and make sure media / image viewing is turned off: Settings - File browser - Use Built-in Media Player (off) / Use Built-in Image Viewer (off). It will also work in the Daijishō frontend if you set the default player to 113 - pico8 - RetroArch 64 - retro8.

note that retro-8 is an emulator, there are a handful of these with various levels of compatibility with real pico-8, but none fully up to date.

The problem on my RG405 with GammaOS was that retroarch only displayed the png and not started the pico-8 game. I have searched for hours to disable the image preview and now it works! You find it here:
Apps->RetroArch()->Load Content->scrolling down to the end->File Browser->disable: „Use Built-In Image Viewer“
After disabling everything works fine and I enjoy the amazing pics-8 games on my RG405 console 👍😄

How did you download the cartridges? Did you just download the .p8.png into a folder and load the directory from the Daijisho front end or is there more nuance to it?