Simply it's when you watch an anime or a movie and/or when you read a manga or any book and any or all of your expectations truns out to be wrong or your favorite character dies in a way that feels that the author is trolling then you my friend got geged.
Example "oh this character is so cool I hope nothing bad happens or this character is amazing and has so much potential there's no way he dies " and then he dies that's a classic example of getting geged