Well... technically the Combine didn't come from Gmod. Gmod is just a modification of Half Life 2 where the combine came from.
GET READY FOR SOME BACKSTORY
After a theoretical physics organisation known as Black Mesa (which contained the famed Gordon Freeman) opened a portal to another border universe known as Xen, creatures from Xen began pouring in and causing a great disturbance. Xen was a border universe, a universe between universes, the creature of Xen fled to this border universe because of a terrifying race known simply as the Combine. Eventually the Combine discovered our universe and the advisors (the leaders and masterminds of the Combine) sought to destroy earth. The Combine is a race of other races that are mind controlled, so the advisors sought out earth to mind control other humans who became the combine soldiers you see in garrysmod. Genetically superior, with a mind of dedicated loyalty to their unforgiving masters, the combine lead the domination of earth and have repressed humanity years, not even allowing them to breed. The weaker people who were captured by the combine were turned into stalkers, their minds replaced with lasers as they work as the main engineers of the combine citadels and facilities.
I hope that cleared some stuff up!