Good to know she was watching the gear, but frankly I think her call could have been more urgent and explanatory, like "Mooney on short final, GO AROUND! YOUR GEAR IS UP!", obviously the time to touchdown determining the urgency. Sounds like she was a bit late for it anyway, and he probably didn't notice her call because 1) he was concentrating on powering down and entering his flare, 2) he mistook the gear warning horn with the stall warning, and 3) most pilots are not accustomed to getting instructions from the tower in-between "cleared to land" and "turn left on Bravo, ground point niner".
I recall once in my early hours of flying complex aircraft getting a similarly calm call from the tower when I was on a quarter mile final, "Arrow 208, have you forgotten something?" As I quickly dropped the gear I responded, "Yeah, my checklist".
I wouldn't think a go-around call needs to be more detailed or urgent than simply "Go-around" no questions asked. I think she handled it well, maybe late but who knows.