I had never heard of griffin before. But I also have no ultimaker printers. Googling it seems to be a worker at ultimaker and they use a firmware named griffin, so probably created by him. But I found no documentation for griffin at all. Might explain why prusaslicer doe snot know the flavour. You might try marlin instead. Eventually you need to tweak the start g-code a bit. But I think the regular gcode is the same so it does not matter that much what you choose as long as set temperature commands match.
