When i first moved to Bangkok it was very difficult to find any information on gyms and I was referred to hotel "gyms" always. Sorry but 1 universal machine and a treadmill does not a gym make.
OK, here we go.....
CHEAP - Din Daeng Thai/Japanese Youth Center - located at Din Daeng near Victory monument BTS. Get a motorcycle taxi and say "pohm bi fitness cup" (I go to fitness thanks) they know where it is. Cost 20B (50cents) a year.
GOOD - Fitness First Thailand - well equiped gyms as per the fitness first franchise, google will tell you thier website and you can find one close by.
BEST - California Wow - Suhkumvit, Pink Lao, Sathorn, Paragon + more - Hammer strength and all the gear you need for serious workouts - this is where I train, also their website (search for california wow bangkok) will give you details on where to find them.
Hope this helps someone, there aer other gyms available in different areas but this will cover your needs until you find your feet.