There is a gas canister in the "Guardia Lacos I" outpost (N 40 40.505, E 5 42.653) in the Lacos Province of Insula Fonte. Shoot your grappling hook at a soldier, then at the canister. Shoot the canister to make it fly into the air with the soldier attached to it. You can also do this on your allies, as it does not have to be an enemy soldier. You can still do it after the outpost has been liberated. The gas canister respawns when you fast travel to the outpost.