You're right, killing more than 3 million people by gas chamber, at times said to be 20,000 a day, just screams nuance.
OK, I'm going to assume you actually don't understand, rather than assume that you're being deliberately obtuse.
Nevertheless, some generalities are in order: You people (by whom I mean Holocaust deniers, although the following would apply equally well to most extremists or people with extreme beliefs — and Holocaust denial certainly applies) seem to be fundamentally lacking in the ability to see things in anything other than black and white. Your "black" in this case would be that the Holocaust didn't happen; your "white" would be the above statement "killing more than 3 million people by gas chamber."
Here's the difference between the way you look at things and the little thing we'll call "the truth." You look at things such that if white above isn't true as you see it, then black MUST be true. People with functioning gray and white matter will, instead, conclude that if white isn't true, then black MIGHT be true, but some shade of gray might also be true. (Of course, it's a complicating matter here that white is, in fact, true, but let's assume for the moment it isn't. NOTE: It is.)
So I mentioned nuance, and you apply that term to the idea of three million-plus people being gassed, sometimes in numbers of 20,000 per day. The problem is that when I used the term, I used it to apply to this statement by you:
if you were too young to work, too old to work, or unable to work you were not registered and killed in Gas Chambers almost immediately upon arrival at the camps.
You did a little sleight of hand there and thought that I wouldn't notice.
NOTE: I noticed.
So anyway, here's what we have as our black and white.
White: If you were too young to work, too old to work, or unable to work you were not registered and killed in Gas Chambers almost immediately upon arrival at the camps.
Black: There was no Holocaust.
Rejecting White as above, you automatically accept Black.
White, as I noted, isn't exactly true. It was true at certain times and in certain places (ergo, the term "nuance," not about the practice itself, but rather about understanding that the practice of mass killing tended to vary over a four-plus-year period and across a continent), but not true at other times and at other places.
So I suggest:
Grey: Some people, if they were too young to work, too old to work, or unable to work, were not registered and were killed in gas chambers almost immediately upon arrival at the camps.
If your question is about the dates when this didn't happen, then I'd say it depends on the camp. I think we can safely say that for four of the camps (the Reinhart camps and Chelmno), White was almost always the case. For Auschwitz, it was the case for a big part of 1942 and 1944, less so 1943. For Majdanek, White applies the least of all, but it still applies, again between early 1942 and late 1944, but not on a consistent basis.
You may wonder why things are not so easily broken down. That's because policy shifted over time and there was a fair amount of lower-level management of protocols of mass killing at the camp level. There was also a bit of a lull in deportations between 1942 and 1944, given that most of the Jews that had been targeted by the Nazis had been shipped off to Poland, with the exception of Hungary, the third largest Jewish population in a country either under Nazi occupation or allied with Germany. They were not deported until 1944, which explains the reapplication gassing upon arrival on a very large basis in the spring and summer of that year.
I've given you a lot of chew on, so I'll leave it at that for now.
