I should think the GSP should be tough enough to deal with a little encroachment, but your best bet would be manual removal, rather than relying on an animal to do the work. You are much more reliable than any animal
I deal with a few different macros that aren't eaten by my emerald crab or Hector's goby (which both mostly feed on filamentous alga like GHA), completely by hand. As far as recommending a tang to eat the stuff you have, I have no experience with that family of fish, but (depending on the size of your tank) any of the more herbivorous tangs should do the trick (though individuals will differ in their taste).