Said half-sarcastically to someone playing a Video Game and complaining that it's too hard. May occasionally appear with respect to other challenging, skill-based activities. Frequently spelled and pronounced as "Git Gud!".
Notably, in the game Hollow Knight the character Hornet (Hollow Knight) seems to say this to the player character, in a certain pivotal scene, despite all other voice lines in the game being in an unintelligible fictional language. While undoubtedly just a coincidence, this has nevertheless greatly contributed to the visibility of this meme in the Hollow Knight fandom