There are lots of good gyms here, it depends on your needs. Most clubs have a gym, so if joining a club is something you will do anyway, go for that. Most suburbs also have local gyms, which are generally good. There is also a gym at Khalifa Stadium, very well priced.
I have ended up with a gym near Tebah Petrol Station, it costs QR400/month, or QR1000/3months. This suits me for bodybuilding, but would not suit a family as it is males only.
Give us more information if you would like better advice!
All the gyms I inspected had a reasonable level of equipment, but local gyms seem to have more free weights and are tailored towards body building.
A very positive sign is that all the instructors I spoke too were knowledgeable and experienced.
Good luck!