Stemming from the phrase "rules of engagement" this term is particularly used in 'all vs all' multiplayer shoot’um up style video games, i.e. Halo etc. The action of gagein-it occurs when there is a firefight occurring between 2 or more players and in the moment one contestant defeats the other and is distracted by the win (while also drained in their life) an outsider from this firefight uses this scenario to their advantage by taking out the momentary victor. This is a prime example of the hunter becoming the hunted.
4 players are playing Halo on the Slayer gameplay type. Player 1 and Player 4 are engageing in battle with one another trading intense shots and blows. Both of their health levels, also known as life, are being drained. player 4 gets a lucky shot with a granade defeating player 1. At this precise moment player 2 comes out of nowhere shooting player 4 in the head. Player 4 just got Gaged.
player 3 may say something to the effect of, "Man! Player 2 keeps on Gagein-it!!" or "man he just Gaged-it"