Has AGP always been a thing or has it taken off with the advent of the internet?
That an interesting question. I guess there have always been cross dressers, who get a sexual thrill out of wearing women's clothes. AGP is the same sort of thing, but worse, if you see what I mean. Apparently, the joke is what's the difference between a transvestite and an autogynephile and the answer is about three years. I don't know if that's true, but it gives an indication of the 'spectrum'.
And yes, I think the Internet has a lot to answer for. It allows these disparate group of men to join forces and legitimise one another by dint of numbers.
Add the Stonewall definition of transgender to include cross dressers, and bingo, you've got a bunch of fetishists all claiming respectability via transgenderism. Not just respectability, but victimhood.
It's not fool proof, but a rule of thumb is if they are late transitioning, heterosexual, and have, historically, used their penis in a 'beneficial' way (i.e. fatherhood, sex) etc, then they are probably AGP.
Banging on about periods, pregnancy, lactation, sanitary towels, mood swings, gynaecology, etc, is a dead giveaway.