I've long since sent the email and I ended up using gymnasiums despite my fellow Orgahs' votes (was going to go with gymnasia but then the counter arguments just became more and more pronounced in my head; also it was a biochemistry professor not a classical studies one so he probably would take gymnasia the wrong way more than the right).

Nice to get some stats on usage that shows gymnasiums is indeed the word of choice in both academic on non academic settings recently though, gives some closure on the matter that I did the right thing.