When Gordon and his friends Bubby, Coomer, and Tommy defeated Benry and had that sweet party at chucky e cheese, they expected to be transported back to black mesa, however, they have ended up in a far different land... As you expect, chaos follows.
Sometimes time travel is possible. Sometimes it goes wrong. Sometimes a futuristic soldier goes on a killing spree to reach the time machine to get back home, and begins an era of video game characters finding their way to Equestria. It happens.