Unfortunately, the Guoman Gym is closing next week for renovations. They're not sure how long for either, some people seem to think 3 months, some seem to think a month.
Most of the hotels have gyms, but they are pricy ($100+ per month, some closer to $200+), the facilities are not really what you'd expect from the cost.
There are cheaper gyms around, if you only want weights, there are some small Vietnamese style gyms around town, and some sports clubs have little gyms attached too.
A friend of mine was complaining about the lack of a clean, affordable, all purpose gym focused on weights last year, and now he's well on his way to opening one up.
I'll try to find out how far he's progressed with the set up...