It turns out I was conflating two different films. The first of these is "Habitable Island"
As part of the Unknown Fathers' brainwashing of Saraksh, the common people are led to believe that they live on the inside of a hollow planet with their star in the center and that this is the entirety of the universe. The main character, Maxim, is native to earth and crash-landed on this planet where he learns from his Sarakshan girlfriend that this is what they believe.
The second film is "It's Hard to be a God"
Here, the main character is sent to a planet that lives in the Middle Ages with the aim of observing the natives. According to cinematext (A website which contains transcripts of movies), the scene with the gas takes place at the very end. There's also the "Self-destruct mechanism activated" part earlier on.