Life Line has closed for renovations but will re-open in September as the "Medical Fitness Centre" ... they're very close to Rotkreuzplatz and fairly good. It's relatively small and quiet ... plus quite a few older people but they tend to stick to the light weights and the recumbant bikes. They don't use the machines I use!
There's also a women's gym right at Rotkreuzplatz ... I used it for a while but found it too small and dodgy (+ over-filled).
There's also Munich Gym ... if you're looking for a little bit of "funkiness" and want to train with people who like to be seen. Well, that was my impression ... I may be wrong.