The immense effort needed for executing even a single person by hydrogen cyanide gassing is highly relevant. While you may yield some savings by cutting corners on safety, this obviously can't go too far even if you train other prisoners as operators for the gas chambers. You won't do much gassing, if your operating crews constantly get killed themselves. There are of course other possible choices for gassing humans like carbon monoxide, for instance, that would pose less problems than hydrogen cyanide. But what can you do? That is what was chosen for the Holocaust narrative and that is a hill Holocaust Affirmers want to die on, so let them.
Note that I recognize that there are scenarios in which "basic" methods like shooting might not be appropriate and the increased effort for more "exotic" methods might make sense. The euthanasia program comes to mind - you probably don't really want to do shootings in psychiatric facilities. While this program in contrast to the Holocaust really existed, the propagandistic distortions here also make it difficult to establish what exactly happened. It seems that malnourishment was an important method used, so essentially also a "basic" method. However, there are also claims about gassing with carbon monoxide.
Actually, there exists significantly more evidence for alien intelligent life than mass killings by gassing - we can point to one example for intelligent live in the universe, ourselves, but none for a mass killing that was done with gas chambers.
But seriously, the comparison is not really appropriate. We don't have interstellar travel yet, so the answer to this question is necessarily highly speculative. When it comes to judging methods for mass killing on our (not so) good old earth, however we can do so based on actual solid data. I note that you did not even try to refute this data.
But the choice of this comparison is highly revealing - Holocaust Affirmers have long ago abandoned trying to prove that the Holocaust happened. They now merely declare, that it is not possible to prove that it did not happen. But a really hundred percent negative proof is not possible in principle. It is of course possible to make the Holocaust unfalsifiable if we assume that all bodies were somehow disappeared without trace, that it is possible to organise the mass killing without leaving any documentary evidence and so on. You can do this with any theory, even, as I have discussed in a post in another section, in Physics: By adding enough "Dark Matter" and "Dark Energy" fudge factors, you can always bring General Relativity back into agreement with observation.
The question to Holocaust Affirmers is therefore: What would you accept as falsification of the Holocaust narrative? About everything that can be falsified has already been falsified!