German cremation technicians call the lower part of the oven, which generates hot air for incineration ‘Generator’. In a letter from Topf und Söhne to the Neubauleitung in KL Mauthausen (Letter from Topf to SS-Neubauleitung KL Mauthausen of 6 January 1941. BK, NS4 Ma/54 January 6, 1941, as cited by Mattogno in ‘Cremation Ovens Auschwitz’), these technicians called the produced hot air ‘Generatorgase’.
To generate ‘gas’ in a ‘generator’ is simply part of the incineration process.
Because the gas-generating process takes place in the lowest part of the oven-building, at a lower level, it is necessarily situated in a souterrain, a sub-level or even a separate floor; in any case: below the floor level of the cremation room.
If so, it’s logical and completely normal to call this Generatorgase sub-level: “Vergasungskeller”.
Also Prof. Arthur R. Butz has argued in the same way:
Prof. Arthur R. Butz wrote:The "Vergasungskeller" (gassing cellar)
Earlier, I considered a widely-cited document dated January 29, 1943, in which Karl Bischoff, head of the Auschwitz construction department, reported to Hans Kammler, head of the SS engineering office in Berlin, on the operational status of Crematory II: 
"The Crematorium II has been completed - save for some minor constructional work - by the use of all the forces available, in spite of unspeakable difficulties, the severe cold, and in 24 hour shifts. The fires were started in the ovens in the presence of Senior Engineer Prüfer, representative of the contractors of the firm of Topf and Sons, Erfurt, and they are working most satisfactorily. The formwork for the reinforced concrete ceiling of the mortuary cellar [Leichenkeller] could not yet be removed on account of the frost. This is, however, unimportant, as the gassing cellar [Vergasungskeller] can be used for this purpose [...]"
In his book, Pressac wrote that my interpretation of the term Vergasungskeller "though perfect in its literary form, was technically worthless."  He interprets the term Vergasungskeller in this 1943 document to mean a homicidal gas chamber, and made this number one in his list of 39 "criminal traces" of extermination gassings at Auschwitz. 
Although my translation of the term was technically correct, I would now say that Pressac showed that, in this case, my interpretation was not correct. However, Pressac's interpretation is also incorrect, as shown by the evidence he himself reproduces. It is necessary to consider this matter in detail. 
The two important German words in this regard are Begasung, treatment with a gas, and Vergasung, gasification or conversion of something into a gas, even in the loose sense; for example, the German word for carburetion is Vergasung. Thus, although "fumigation" should normally be "Begasung," for no clear reason German often allows "Vergasung" to substitute for "Begasung." Thus, gas attacks in World War I were referred to as Vergasung, and professional fumigators often speak of their operations as Vergasung rather than Begasung. However, it appears that Begasung never substitutes for Vergasung and that a fumigation or delousing gas chamber is normally a "Gaskammer," not a "Vergasungskammer" or "Vergasungskeller." Accordingly, the delousing gas chambers at Auschwitz were called "Gaskammern."  These are the sorts of arbitrary conventions of usage, not deducible from a dictionary, that occur in any language.
Despite all this, the normal meaning of Vergasung, in a technical context, is gasification, gas generation, or carburetion. In view of that and knowing that some cremation ovens were of a design requiring a combustible gas-air mixture to be introduced by blowers located outside, I interpreted the Vergasungskeller mentioned in the 1943 document as a place where coke or coal was converted into a combustible gas, mixed with air, and then introduced under pressure into the cremation ovens.