I head many rumours then ive verified on tv news of people forgetting just slightly their propan gas open and died as a family. As for nitrogen or helium suicide they work better as you dont feel the annoying odor propan-butane gases have. You can simply die even unintentionally with odorless gases like Nitrogen and helium.
About what type of gas in the room you mixed it a bit. Do gas MASKS work no matter what type gas in the ROOM?mask or room method ?
As for room method you need to search in google depending your chosen gas if its heavier or lighter then air. Accordingly youll know if that specific gas will accumulate on the floor or the ceiling part of the room so you know where to stand. As for mask method just making a correct mask will work with any of those 3 gases. You can even work with CO gas emmited from burning charcoal . It needs to be burned red hot ( not producing fire but red hot) and a lot of them. So they eat up the oxygen of a sealed room where no gas exchange happens.
This will eat all the oxygen room and you fade out and die without pain or odors or anything ( if you cant find propane,nitrogen,helium)
propane is the most common and used in kitchen ovens to cook food so i doubt you wont find it