I would like to recommend the gym were going, Life Gymnasium in Holiday Inn. Reasonable price, good trainor and staff. Think they have a promo right now. Equipments are ok.
The gym at the Yachty is about K600 per year and the gym at the Aviat is a little less than that. The Aviat also has Squash Courts (2) and a great pool for a cool down. Some compounds (eg: Era Dorina) have a small gym onsite.
You can also use the gym at Airways on a day rate (75 kina off hours, I think), if you want something that you don't have to be a member for. They also have full memberships and classes, etc.