When the enjoyment one used to feel when playing video games is stripped away by a subconcious pressure to do something else. A person with gaming fatigue will often load up a game, only to close it within a matter of minutes for no reason and stuck staring at their massive game library complaining that there isn't anything to play.
Me: I can't wait to load up this new game I bought
10 minutes later...
Me: Welp, i'm bored so I guess I'll go play something else
Browses massive game library...
Me: There isn't a damn thing to play! Even my favorite games are boring
Me: I hate gaming fatigue! Guess i'll go jack off and cry myself to sleep.
After going through the development cycle for years, game testers grow tired of the game they work on and often times no longer have the desire to play the game.
Joe: "Hey you want to play this new game that came out? Its awesome"
Tester: "No thanks, I have Tester Game Fatigue. Spending 3 years on every little detail for long hours has sapped my desire in playing the game."