The term refers to the meme trend, where people want to obtain a goth style girl as their girlfriends.
The catch is, that in all images, “goth gfs” are pictured as very rare and “sought after” girls, who anyone would fall for, but in real life, “Goth girls” don’t look as good or are as easy to handle as thought.
The first recorded mention of the topic was posted to Tumblr as a made-up conversation with the title “me and my goth gf”: B: “bb ur hands r so cold” GG: “that me soul.”
The whole thing picked up from here and spread wildly on the internet as people were very fond of the idea of having such a girlfriend of theirs.
You could say it has created a new fetish on the internet.
After the original conversation with a “goth gf” mentioned, the trend went crazy and people created multiple images and made-up conversation on message boards and image sharing sites.
So far crazy that it has even spawned a subreddit solely for the purpose to share pictures of very hot and/or naked goth women, which has accumulated over 264K members in just 2 years.