Depends on the concentration. If you introduce it slowly, the target simply goes to sleep and doesn’t wake up. Gas them too fast and they’ll feel like they’re drowning, which is one of the most terrifying ways to die and will trigger just about every survival instinct available. You frequently see this with animals at particularly large and busy farms: the gasmaster wants to get as much work done in a given period of time as possible, so he introduces the CO or CO2 (usually the latter) very quickly, at high concentrations. While this kills the target faster, it also makes them die in agony, which is terrible.
The Germans in WWII also had this problem with their mobile gazwagens at Chelmno. These were essentially 4x4 trucks with the exhaust hooked up to a sealed box in the rear, where victims would be locked up. In theory, this provided a “humane” method of extermination, and more importantly it would also in theory spare the killers from the psychological anguish of having to shoot their victims. The problem with this (aside from it being genocide) was that early on the gas flow was inadequate, and did little more than cause brain damage. This in turn caused the drivers to rev the trucks very hard in order to stop the screaming in the back, which they hated; and this became a sort of nasty cycle where even after the gas flow was solved, the driver would still rev the truck extremely hard and introduce too much gas at once, which caused the victims to panic and scream and shit themselves, which caused the driver to panic and hit the gas harder, etc.
That’s your bit of horrible history for the day, I guess.