The Gas Cylinder truck comes to our area at a fixed time everyday from 4-6pm. You can ask a nearby restaurant to see what is their routine time in your area.

I know of two Gas refilling shops, one is at the intersection of Mansura St and Rawdiyaat St near the roundabout.
The other is near the airport. If you cross FFC and drive further on towards Wakra, then turn right at the first signal and drive straight. The Gas shop will come on your right side near the mosque. We bought our cylinder from that shop (Qatar Gas).
Hope it helps.