don't want to use poisonous gas to commit suicide
Inert gas is not poisonous. You die because the inert gas pushes away the oxygen and you need that to live, not because of the inert gas itself. Big difference. An inert gas often chosen for this method is nitrogen because it's cheap, easy to aquire and plentiful. The air you breathe right now while you read this is ~78% nitrogen. It doesn't react with you. It's just kinda there. Hence inert. It's good that our atmosphere is mostly inert gas because oxygen is very reactive and there would be probably massive fires constantly otherwise, which for a part of Earth's history, was likely true. When correctly done, you don't notice anything as your body has no good way to detect the absence of oxygen, just the buildup of carbon dioxide, which when correctly done, this method also prevents. You feel like you're breathing normal air but actually are dying of hypoxia. There's videos on youtube what dying of lack of oxygen in e.g. great altitudes is like. It's quite insidious and you're not really aware you're dying.
In the US, recently the first inert gas execution was done, with nitrogen. It did not go smoothly because naturally, the guy panicked and apparently tried to hold his breath, which would lead to carbon dioxide buildup in his blood (the one thing your body *can* detect) and probably triggered a hypercapnic alarm response. IMO, it's excessively cruel to murder somebody like this. But as a suicide method, it can be very peaceful and quick, big difference being that you choose to move into the oxygen-free enviroment voluntarily and don't try to hold your breath as long as possible. The
Sarco pods that got quite a bit of attention also work with inert gas.