It is painless. You can try it out yourself. Your body only detects excess CO2 not lack of oxygen. You will go lightheaded before passing out and eventually dying of oxygen starvation
This is easily the most dangerous method possible for everyone involved. Strangely enough our body only reacts to buildups of CO2, it does not react to a lack of oxygen.
So while in theory it would be painless as an execution method, if anything leaks into the viewing area for the execution there is no visible signs anyone is in danger as oxygen is being displaced thus potentially killing everyone involved.
There's a whole field of industry for gas detection, whether it be for the presence of toxic or explosive gas, or the absence of healthy breathing air. It's an easily mitigated problem. If the detector on the wall goes below 20% o2, an alarm sounds, circulation fans turn on, and everyone leaves the room.