1. Start up the game, go to the devconsole (typically opens when you press tilda "~" key on the keyboard) and execute (from here onwards execute means "type in text that follows line by line into the console and press Enter on the keyboard after each line):

2. Let the map load and then execute in the console:

3. Rush effortlessly through the level to a point where the entrance to the secret passage is expected to be located. Check section "[ENG] How to get to the place? Where to install the "Gas Tank"?" in this wonderful guide to grasp an idea on where to go. Or view the following video at around 1:07:00 time:
https://www.youtube.com/watch?v=L33GDeowy_g&time_continue=4021
Important: Note that passage most probably would be blocked for you but no worries, it won't stop you from getting into it.

4. Open up console and execute:
Now you're in "no clipping" mode and will be able to fly through the wall/rocks that block the entrance to the passage into the passage. As soon as you position yourself inside the secret passage go to console and execute commands from last code block once again to turn "no clipping" mode off.

5. Follow the secret passage to a point where the socket for the gas canister is located:
https://steamproxy.net/sharedfiles/filedetails/?id=1824713062
Then make sure you are looking directly at the nearby wall or at the floor right behind you (i.e. not at some distant point in the cave where Gonarch is sitting) and execute in the console:
Close the console and you will see a gas canister spawned right at the point your crosshair had been pointed at. Pick up the canister and yet again open open the console and execute:
Close the console and create a new save. Exit from the game, run it again and load up your save. Important: achivements probably won't register if you try to get them without save-quit-run-load cycle.

6. Put gas canister into the apparatus and push the button that had turned green. Watch Gonarch die and get your beloved "The Plan" achievement.

7. Reload your save but this time do not put gas into the apparatus. Instead follow the ledges on the left side of the cavern using long jumps and proceed to the next level leaving Gonarch alive behind. Get the "Unquestionably Ethical" achievement in process.

Tested this approach to work with latest public beta as of 13/02/2020 (it is final 1.0 release candidate beta). It is a big timesaver for greedy achievements hunters out there that allows to get these two in an almost effortless way. I don't recommend following this one though if you want to get all the joy of actually accomplishing this in the hard but intended way.