Mild spoilers. Do not read if not wanting to get spoiled.
This mission can get really nasty.
It is either too difficult, if you go all hostile before using gas, or gets too time critical if you go with the gas.
But maybe I figured a way how to resolve it with the least stress.
I activated the gas and just waited and waited and waited, until all combatants in first area became Mutants.
Then I realized there is still 1-2 tiles free passage without gas in the corridor, and that gas will not fill every space in the map. But it can get dangerous since gas sometimes randomly expands for single tile, but it can also retract from those same tile, making travel possible.
This should make it possible to pass the corridors and get the keycard on one of the shelves. And there should be at least one tile without gas to proceed to the south area with more guards. Just watch for gas to not randomly expand where you want to walk..
Note that if you do not get this card and do not have good electronics, and progress to the next area, it is a game over, since going back to this area without gassing yourself is impossible.
Now, I'm not sure about speed of gas progress in the second area, and if it is in any way tied to the gas progress in the first area. But even there is should be possible to have 1 tile corridor (with random expansion/contraction of the gas), to be able to pass.
Only gotcha, end game moment, is getting to this area without electronics or keycard, since it is not possible to go back without being gassed on map transition.
