How to season iron cookware on gas stove?
I strongly advise against doing it. I tried stovetop seasoning at home and got terrible results. 

A stove gives you hot spots - on gas, this will be the ring where the flame touches the metal. The temperature of the metal in this hot spot is way too high, and the oil burns instead of polymerizing. You get some oil-charcoal in this place, which doesn't have non-stick properties, and flakes off after a few uses. 

Outside of the hot spots, the temperature is not high enough. The oil doesn't polymerize thoroughly, and forms a sticky paste instead of a smooth one. Your food will stick to these parts of the pan even worse than to the charcoaled parts. 

Conclusion: use an oven. If your oven is too small for a pan (I only have a toaster oven, 30x30 cm, and my pan+handle is way too long even for the diagonal), leave the door cracked and seal the crack with alu foil. This is not very energy efficient, but you only do it once per pan. It worked for me, and I got a real, non-stick seasoning after multiple failures on stovetop.