Garry is a YouTuber who is a spin off of the meme "Larry vs Barry", where two channels Larry and Barry had a battle in the style of PewDiePie vs T-Series.
Garry saw this and made his own version. "HELP ME DESTROY LARRY AND BARRY". After a user on reddit saw this, they made a meme, which blew up on the Dank Memes subreddit gaining over 10,000 upvotes.
After this, he continued to gain popularity with Instagram meme accounts such as @Flame.v1 (flamedotv1). Because of this he managed to gain over 1,000 subscribers in one stream.
This went on for a while before Garry decided to stop only after about a month, in this time span he had gained over 21,000 subscribers and once he hit 10,000 had a Minecraft duel with Barry.