It looks like a beautiful little coin under that stuff. If it were silver I would start with a lemon juice (citric acid) or vinegar (acetic acid) bath since the crud looks like a dark carbonate deposit. There is no danger to the coin from this since gold is chemically inert. However Gold being a relatively soft metal the problem
comes from applying any kind of force or friction to further loosen the deposits but which might scar the surface. Perhaps long soaks followed by gentle rubbing with a wet cotton Q-tip will gradually loosen most or all of it. Keep in mind that you can soak the coin for as long as you have patience since the acid will not harm anything.
It looks as if whoever
had the coin before you may have removed a lot of the deposit already, leaving behind the hard parts stuck in every little crevice?
