Finding a decent Gym/Health Club in Nizwa is quite challenging and require some time for researching about the best possible option that suits one's budget and provide a convenient workout atmosphere.
Here below summary of the possible options for Gym/Swimming Pool Facilities found in Nizwa Area:
Golden Tulip Hotel in Hai Al-Turath, is a pricey option. Their membership is subject to management approval, so all you need to do is to pass by the hotel to collect the application form from the Hotel's Front Office to fill it up and submit it for approval. For single person charges will be as the following:
1 Month = OMR 30
3 Months = OMR 60
6 Months = OMR 100
12 Months = OMR 175
Blue Dragon Gym in Farq Area (Behind BBQNation Restaurant), that is a male only Gym. Quite cheap and has basic set up. The downside of this place is the location, its located in the basement of one building with zero natural light; If that is of no importance for you, then this is an ideal solution for a place to work out.
Nizwa Sports Complex - The Complex offer a Swimming Pool and Gym, I didn't get the chance to check it out yet due to their working hours which does not match with mine but I have read some good reviews about it.