I heard "The Man who Folded Himself" by David Gerrold was an early sci-fi example of TG, (namely Rule 63) written in 1974. The story involves a time-traveler fucking himself when he meets another version of himself from a timeline where he is a girl.
I haven't gotten around to reading it, but it might be interesting and it's unfortunate that copyright lasts so long after a book goes out of print.

The other stories recomended by wikipedia have much more TG. Namely, in "All You Zombies" the main character actually turned into a girl and then was fucked and impregnated by himself. Sadly, it was written in 1958 when all media was tame and censored to appease the moral guardians, and the idea of blowjobs wasn't even common knowledge, and so there was hardly any sex. Not even though sci-fi was avante garde and a medium exploited by writers who wanted to avoid censorship in both the west and Russia (Solaris.) And yes, I know that a few years later Heinlein wrote a lot more sex in "Stranger in a Strange Land," but it was also banned from schools reading lists.