This Is What Happens When Gordon Meets Chell
It doesn't matter that Gordan Freeman's timeline doesn't quite match up with Chell's. The two share a common fictional universe, and that's more than enough to allow them to meet. I admit, I don't remember Chell's Portal gun having the ability to evaporate enemies, nor do I remember Freeman's gravity gun absorbing shrapnel to fire off a gigantic bullet, but it doesn't matter. It's fun.