(X-Post from r/fantheories) Gmod is cannon in the Half Life universe

The entirety of the half life universe is actually much more based around creatures known as Gmen. The king gman, named Garry, created a program that lets all of the Gman create there own games. The Gmen are incredibly powerful creatures, so this is what they do to keep themselves from getting bored. One perticular Gman, named Gabe created a very popular game known as half life. He starts a trend after this game, where he puts himself in the game every now and then as a joke. Another Gman, named Glados (The Gmen have very unique names) Puts himself in another game, but pretends to be a female robot for the hell of it. He often times makes refrences to Gabe's game.

TL;DR Glados and Gabe Newell are just bored Gmen who created games using software created by a Gman known as Garry.