Below story: deals with using inert gas for state executions, for example Argon, nitrogen or a mixture of these two that has been tested and researched for use in the meat packing industry very successfully.
https://www.cnn.com/2024/01/25/us/nitrogen-gas-execution/index.html

https://www.theguardian.com/us-news/2024/jan/26/what-is-nitrogen-gas-execution-risks

https://www.economist.com/the-economist-explains/2024/01/27/why-execution-by-nitrogen-gas-is-so-controversial

Condemned Prisoner Holds Breath During New Execution Method | Kenneth Eugene Smith Case Analysis
https://youtube.com/watch?v=u54fi3PaC_4 [Embed]

The problem with a human should be obvious, since a human knows that they are being killed they will need to be given an injection to induce sleep before the gas is turned on, or you can watch them struggle, bring your popcorn.