As Thomas Kues has showed, Treblinka was reported to be exterminating Jews with gas before it had begun (according the standard holocaust story) to act as an extermination camp – despite Nick Terry’s claim that the document demonstrating this was a revisionist forgery. Interestingly enough, it seems that the same may be true of Majdanek.
An article in the May 15 edition of the “antifascist” Warsaw ghetto publication Der Ruf referred to the killing of Jews with gas in the Warthegau, and more recently in Lublin (not to mention the “unbelievable ‘krematoria’ for living people in Belzec”)
This article was published before the Majdanek (delousing) gas chambers were built. Thus if we interpret “Lublin” as a reference to the Lublin camp (i.e. Majdanek), we have a case of a premature Majdanek gassing claim.
There is however another possible interpretation of the passage. Another article in the same edition of Der Ruf contained a reference to the “poisoning of old people and children with gas” just after mentioning a number of cities where Jews had suffered, Lublin among them. This suggests the possibility that the killing with gas in Lublin was supposed to have taken place in the city itself. Such stories did exist – some rumors recorded that August by the Swedish consul Karl Vendel contained a reference to gassing in the cities:
In the cities all Jews are gathered; they are officially informed that it is for the purpose of ‘delousing’. At the entrance they have to leave their clothes, which are immediately sent to a ‘central warehouse textile materials’. Delousing is in practice gassing, after which all are packed into previously prepared mass graves.
The statement in Der Ruf about gassing of children and old people seems to be loosely (but certainly not exactly) reflected in a statement from the Vendel document:
The number of Jews murdered in Lublin is estimated at 40,000. The Jews over fifty years of age and children under ten are especially subjected to extermination.
Ultimately, I would say that it’s unclear whether the gassing in Lublin mentioned in Der Ruf on May 15, 1942 was supposed to have taken place in Majdanek or in the city itself, but either way, the story is yet another false rumor of gassing.
 Der Ruf, May 15, 1942, reproduced in Polish translation in Biuletyn Żydowskiego Instytutu Historycznego, No. 5, 1953, pp. 9-11.
 Der Ruf, May 15, 1942, reproduced in Polish translation in Biuletyn Żydowskiego Instytutu Historycznego, No. 5, 1953, pp. 4-6.