I used RiscOS as a development workstation for an ARM-based settop box in the late 90's. I remember it as being very fast for its time, although the UI was a little weird. It was at the time where Mac, Windows, and X Windows did things like window sizing and copy/paste just differently enough that if you had to switch between machines, your head hurt. I may still have the workstation out in the garage from the project.
I'm not sure how suited to the Gemini it would be --- it was very mouse-centric and wanted a big screen. Looking at the source code might be fun, though.