when the gas cloud picks a target, that palyer gets a 10 stack, and every time the debuff ticks, one stack is removed, the debuff ticks every 2 seconds so after 20 seconds the stack is gone, and it will pick a new target. you want the gas cloud to be dead before it picks that new target. as for kiting vs simply remaining in position is completely dependant on your DPS, if your DPS can kill it before the cloud reaches its target, then you don't really need to kite, but that means your DPS have to be really on the ball, because if they fail, the focus is going to be cornered and will get hit (as the best place for the focused person to be if they aren't kiting it is in the north corner).
2) I didn't really get the mechanics about the gas clouds. When they spawn a random member will get 10 stacks, then 9, then 8 etc? until the stack is gone? or until 20 seconds have passed and they switch to another target? Is it better to kite the gas cloud while the dps nukes it to prevent it from reaching you, or should you remain your position? So the main thing is to actually kill it before 20 seconds have passed to prevent him from switching to another target?