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Try warming it up a bit first I think that loosens it up a bit, melts the glue or something. Best way to get it off is it get it with a Mole Grip and twist it off, but that usually damages the knob quite badly as there's quite a lot of force needed.
That's speaking from experience with the gear knob in my 1.2, I don't think the 172 one should be much different.