If you have any way to expand your vision radius (night vision, flashlight, etc) you can use the X/look-around key to scan the floor: slopes are considered base terrain and will show up under the gas. You can also drop items on slope tiles as item(s) take display-priority over gas.
But yeah, infinitely expanding gas is the bane of mine-exploration. Even with Full Night Vision and a gas mask, it’s still a PitA (worse if there’s other underground areas nearby, so the gas vent gets to happen even though you’re not in the mine).
Good luck; you may well need it. 
