Hmmn, I think you need to consider revising your question (if possible or there's a mistake) because "act" is NOT a gerund, it is a WORD WITH MULTIPLE MEANINGS.|
If you meant "acting" (because for gerunds, we have root word + -ing), we have:
Mike loves acting on stage.
(Loves what? Loves acting.)
WRONG: Mike is acting on stage.
("is acting" is a VERB PHRASE, not a gerund.)
Hope this helped!!! :D