Gemmer would indicate it is a person who gives gems, like, say, a templar.
Gemmed would indicate a person who has been given a gem, like, say, a witch.
Doesn't have to be so ultra-literal. A nutter isn't one who nuts, a rinther isn't one who rinths, a necker isn't one who necks, a Stormer isn't one who storms, and a magicker isn't really someone who "magicks," although you can sometimes coerce "magick" into a verb. Looks like this "-er," as distinct from the "doer type -er," is pretty versatile as far as the kind of relationship it can signal, and in this use, it seems to have a vaguely derogatory feel toward the person mentioned. Perfectly reasonable system, and I'm tickled pink that Zalanthan language has developed characteristics distinct from everyday English, where this same meaning is not nearly as widespread (I can only think of "nutter," but there are probably others.)
(Conveniently, one would never refer to a Templar as a "gemmer" -- except maybe in some very specific and contrived context -- since that implies gemming mages is their main function, and that's not socially acceptable. So there is no room for ambiguity. "Gemmed" seems more formal and elegant in contrast, so we're probably just talking about a difference in spoken style level.)
