Just because this is probably the most off-the-wall item I've seen here in quite a while, I spent a few minutes researching and the following generated itself:
Mimaki, a very vertical market manufacturer in Singapore, appears to have run it products on and written software for Windows OS only.
Try as I might, I cannot find any reference to a "Mimaki Pro series GC-5" anything.
There IS a long-obsolete model CG-5 plotter which I'll have to assume is what you're talking about. Hopefully you've been searching for that and the "GC" reference here is just a typo.
There doesn't however, appear to even be a Windows driver anymore for that product, let alone a Mac OS9 one.
So, when you say: "surely this exists somewhere?", I can only ask: Why in the world do you think that?
Again, I find zero evidence there was ever a Mac OS-anything for any Mimaki product.
Admittedly, possibly stranger things have happened, but I think you're chasing a ghost.