<wow. I can’t even remember the word “gay” having been appropriated by homosexuals as early as the early 70.
There is a scene in a movie, I forget if it is ‘Philadelphia Story’ or ‘Bringing up Baby,’ with Cary Grant answering the front door in a frilly ladies’ bathroom. The person at the door comments and Grant says, “I went gay all of a sudden.” If it was in ‘Bringing up Baby’ that would be 1938.
Perhaps it wasn’t in common use, but put a man in a frilly bathrobe and I think most people would get what he meant, whether ‘gay’ was in common use or not. I remember seeing that scene and being quite surprised that gayness would even be referred to back then.
There were unquestionably subtle homo references and allusions back then, especially in the pre-code (pre-1934) films, before the Hays code. However, I don’t think that was the case with the example you cited, as “gay” would have been meant in the wacky/balmy sense... a guy in a woman’s bathrobe being akin more to waking up after a night’s wild-party, drunken silliness like someone wearing a lampshade for a hat.
A woman’s bathrobe as a humor device (like the time in the “Blondie” comic-strip in which the doorbell rings and Dagwood jumps from the bathtub and into Blondie’s bathrobe to answer the door... prompting the salesman to say “what a homely dame”) pops up every now and then, and generally wasn’t a ‘gay’ gag.