On my Psion 5mx, I found the safest way to remove the keytops was with a big blob of Blu Tack (or similar putty-like adhesive). Press it firmly onto a keytop, then pull up sharply; if you do it right, the keytop comes away with it.
I don't yet have a Gemini to test that with, but perhaps someone could try it and report back?
(Blu Tack is also great for removing dust and other gunk, especially from crevices you can't easily reach.)