I can't speak intelligently on what is or what is not Goju.
I will say this. Okinawan karate in general does address all ranges of combat. It would be ridiculous to assume that the old masters never considered what happens if you should fall accidently, or be taken down. Would they have just turned turtle and quit fighting??
The answer of course is NO WAY!!
I try to stop thinking along the lines of technique and more along the lines of principles
Certain principles that you have learned while standing or in the clinch will also work on the ground.
Also it is important to remember your paradigm. While Okinawan Karate in general has answers for what to do should you be taken to ground, it wasn't really designed to stay
Accidently fall, get swept or taken down... defend/release and get up!!
From a modern point of view, I'll say this. If your style in general does not at least address being on the ground, or your instructor never learned it to show it to you, You definately have a responsibility to supplement. Makes no difference whether you use som BJJ, wrestling,or other art.
Is it still Goju or American Goju or anything else you care to call it?? Sure it is. You just give your students another tool in the bag.