Oh my gasssh is a stretched spelling of oh my gosh, a long running exclamation of surprise that creators turned into a short reaction sound for short form video. The elongated version spread through meme edits and soundboards because the drawn out ending is easy to recognise and easy to time to a visual gag. It is now used less as a genuine expression of shock and more as an ironic cue, signalling to viewers that a reveal, mistake or awkward moment is meant to be taken as comedy.