These are ridiculous lies propagated exclusively by Holocaust deniers/revisionists, Nazi apologists, Stormfront and other assorted white supremacy philosophies.

We have an extensive segment of our FAQ that deals with Holocaust denial, so I would direct you here. This AMA Panel is an especially good place to start. If you want a more recent thread, I would recommend you go here.

TL;DR is that yes the Holocaust happened, yes it was roughly 6 million Jewish victims killed (As well as millions more non-Jews), no the gas wasn't for delousing, and I can also assure you that Hitler really does deserve his reputation. Hope that helps.