Banus is one of the original gyms in Kathmandu, been around a long time. I havent been there recently but it used to be good (more than 10 years back!)
Fitness Mantra is the one I go to now. Its around Rs4,000 a month or Rs 11,000 for 3 months. Packages available if you only want cardio or only want weights.
There is also Rage in Naxa - I looked at that one two years back and a few weeks ago... did not like the staff attitude on both occasions . Apart from that Fitness Mantra has better equipment and is located on a quiet road.
Good luck!
Jackie