if you want the best of the best i would recommend golds gym but, it is very expensive and sometimes does get crowded, the average cost would be 1500 qr for 1 or 2 or 3 moth membership time and with that they force a diet course ( nothing special just a summary that u should eat protein and not fat and water)

if you already now the basics and do not need a trainer to introduce you to everything, i would suggest the gym at nero bianca, in matar qadeem, airport road next to the pizza hut in airport , cost is 400 qr for 2 months, other daily monthly or yearly options are also available, the only problem is that it is a bit small and some equipment like the pullback/rows is sometimes always busy.