In my answer I'll rely on this entry previously written by me http://holocaustcontroversies.blogspot.com/2017/05/rebutting-twitter-denial-most-popular.html#flimsydoor where you'll find lots of photos and references.

1. First of all, here we're talking specifically about the gas chambers of Auschwitz. You can see some surviving metal gas chamber doors in some smaller camps (however, those gas chambers didn't play a significant role compared to Auschwitz, so there's no reason to dwell on them).

The Aktion Reinhardt camps don't survive, but at least in some of the smaller gas chambers there wooden doors were used temporarily, until the bigger and better GCs were built, as Erich Bauer, who helped to build the Sobibor gas chambers, testified:

"The doors were indeed wooden; they were changed later, when the gas chamber was completely rebuilt. The airtight doors arrived only later; I collected them myself from Warsaw, but that was not until the new building went up."

http://holocaustcontroversies.blogspot.com/2011/12/belzec-sobibor-treblinka-holocaust_3293.html

This was however a small issue, since not HCN (like in Auschwitz) but rather CO from the gasoline engines was used, any possible small leaks would have immediately dissipated without any danger to the involved.

As far as Auschwitz is concerned, it is necessary to note that none of the doors on display in the small crematorium in the main camp are original gas chamber doors due to the crematorium having been converted into an air-raid shelter already in 1944 (by that time the provisional gas chamber there had long ceased operation, the absolute majority of the mass killings took place in Auschwitz-Birkenau), and then was partially reconstructed after the liberation, so you can see a confusing mix of flimsy wooden non-gas-tight doors on the inside and air-raid shelter doors on the outside, none of which however tell us anything about the original state of things.

The dishonest deniers of course try to use this confusion for their propaganda, instead of clarifying the facts.

None of the Birkenau gas chambers survive, they were destroyed by the Nazis themselves and only ruins can be seen today.

2. So how do we know that wooden gas chamber doors were used?

First of all, at least one gas chamber door survived, we have post-liberation photos of it:

https://2.bp.blogspot.com/-a-nd8aohCnM/W2yx6dc3hWI/AAAAAAAAA_U/N22ZNuqvQCgh5uz0Wv2Wqxhq24-fC1bIwCLcBGAs/s1600/486.02%255B1%255D.jpe https://4.bp.blogspot.com/--donjKUBwbA/W2yyDIW5epI/AAAAAAAAA_Y/1LmXtwsS9jgxaiT3qGq2Sp0QZM-QOOAGQCLcBGAs/s1600/486.03%255B1%255D.jpe

Note the heavily protected peephole on the inside of the door.

We also have documents that specify for some of the gas chamber doors that they should be made like the delousing gas chamber doors, and the delousing gas chamber doors in Auschwitz were wooden (see the very first link in this reply for the photos).

3. The deniers like to complain that the wooden doors couldn't have been gas-tight.

But already from the point I made above it should be clear that this is a nonsense argument, since the delousing gas chambers also had wooden doors, which of course had to be gas-tight, especially as concentrations of gas and the durations of gassings in the delousing chambers were much more significant than in the homicidal ones.

Moreover, numerous German documents explicitly call these doors gas-tight:

The Bischoff letter (see 1st link):

"At this occasion we remind you of another order of March 6, 1943 for delivery of a gasdoor [Gastür] 100/192 for morgue 1 of crematorium 3..."

Krema 3 inventory:

https://phdn.org/archives/holocaust-history.org/auschwitz/19430624-inventory/

"Gasdichtetür" - gas-tight door.

And many other mentions: https://phdn.org/archives/holocaust-history.org/auschwitz/pressac/technique-and-operation/pressac0438.shtml

So yes, wooden doors were made gas-tight with felt and served their function perfectly both in the delousing and in the homicidal gas chambers.

4. When cornered about this, some deniers whine about how the wooden doors wouldn't have been able to withstand the crowd pressure.

It is hard to understand why a reinforced wooden door would be any worse than a metal one, and frankly, for the argument the deniers make about the crowd pressure the material doesn't even play any role, since if the deniers were right, metal doors of the air-raid shelter type would have been also easily destroyed.

But of course this argument is just as preposterous since a crowd of 1000 people in a gas chamber is not the same as the pressure of 1000 people on a single door. In fact, had such a pressure existed, the people near the door would have been crushed first and would in fact become an obstacle to any further pressure.

This non-issue is further examined in depth here: http://holocaustcontroversies.blogspot.com/2013/10/viewers-guide-to-auschwitz-surprising.html#doorpressure

As /u/sergey_romanov has written on their website here and also dealt with in the Van Pelt Report for the Irving v. Lipstadt Trial, the doors were wooden but made gas tight by felt stripes. As Henryk Tauber, who had been forced to work in Auschwitz described:

It was a wooden door, made of two layers of short pieces of wood arranged like parquet. Between these layers there was a single sheet of material sealing the edges of the door and the rabbets of the frame were also fitted with sealing strips of felt.