He found her on a trans dating site and it was shown on screen but I find it pretty crazy that yall didn't clock the gf as a dude, or at least maybe a dude, right out of the gate.
Wife and I finished it last night and both enjoyed it. Definitely not for everyone.
As others have said, it is really dark and has a lot of messed up parts. In general we thought it was great acting and hopefully it helps someone get the mental health help they need.
Interesting from IMDB trivia:British news reports say the real "Martha" has come forward (was interviewed by Piers Morgan) saying that Richard Gadd was the actual stalker. She says she's been revealed by internet sleuths who she says have been threatening to kill her. She is not named, but says she will be filing a lawsuit against Gadd and Netflix, despite never doing anything when the play was produced and playing nearby.