Using a gas cooker to die used to work very well in those countries in which the supplied gas contained a lot of carbon monoxide, such as England before about 1970. You didn't even need to think about overnight timescales. You could just put your head in the gas oven, with the supply switched on (but not burning), and you would die quite quickly. But most gas supplies today don't contain any carbon monoxide at all, so it won't work. You certainly don't want to let a large supply of unburnt gas get into your house. Even if you like the idea of dying in an explosion, your neighbors probably won't. The amount of carbon dioxide produced by a gas cooker burning gas normally probably wouldn't be enough to cause death unless you are in a fairly small, airtight room. So I think you need to look for another method.