Gah! is a common onomatopoeia used to express surprise or annoyance. It debuted in Easy Allies lore during one of Kyle's Fun Game Monday streams in which he played Psychonauts, where the expression was prominently used, much to his amusement.
In another stream where he played Beastly for the Wii, he noticed that the background sound ambiance had someone saying "gah!", which was very evident due to the recording's short loop, making Kyle explode in laughter once again.
Lastly, during For the Frog the Bell Tolls stream, there was a scene in which the main character enters a nightmarish state and has a vision of his rival to which he reacts with the same expression, forcing Kyle to pause for a moment and making Chat go wild.