It's all about the technology. Gather round children and I will tell you a little story.
The sun is rising on a brand new day, and the combine are pissed off.
The combine are like "Sheet mangs, we need local teleport tech." They could be so much more efficiant! So they go and call up one of their inter-dimensional advisors (Pun un-intended.) That happens to be the Gman's 'Employers'. They are like, "Whatever you say Combs but we want something in return you deal?" They deal. Lets assume that the Gman's Employers know about humanity before the Combine do. Perhaps they know about them from Black Mesa's Survey teams in Xen?
Anyway, they put the combine in touch with the Gman and he acts as their advisor and contractor. The Gmans like "So whaddya want." The combine explain they need local teleport technology. They dont list a reason - and they don't need to. This is business. If the price is right nobody gives a crap.
So the Gman goes and does all his paperwork - paperwork being setting up Black Mesa with a bad crystal and possibly manipulating the science teams to ensure the anti-mass speccy-magic-laser device has more than enough juice that would create a breach between Xen and Earth - and he succeeds. Now all the combine need is a rocket in orbit with a particular satellite system. Problem is, humans have contingency plans and shit. Damn, now the military are all over Gmans hard work - but wait a moment. Who the hell is this guy? Some ginger-pony-tailed weirdo is killing EVERYTHING. So the Gman's all like, "Aight" and he manipulates the scientists to direct Freeman to the rocket pad and launch it. Freeman deals with that situation so well that Gman's employers begin to raise eyebrows.
Something comes across the minds of the Gman's employers. You see over in Xen land, theres this gigantic flying baby which shoots portals. Local portals - Local to Xen. (Ephiphany!) But anyway, the combine evidently tried probing Nih' and all the rest. They need science - not nature! Besides with him gone the Gman and his employers could control Xen. So it was settled the Gman had nothing else to do around this time anyway, so we went to work manipulating paths, making Freeman travel to Xen to kill the big baby. Should he succeed, the Gman's company get to keep Gordon and would inadvertantly control Xen. A bonus!
So Gordon does his stuff and gets put to sleep by Gman. Fast forward however many years it was to Half-Life 2. AND GODDAMN IT MAN, Turns out the combine wanted to keep Xen! But Gman's employers never thought to ask - they also wanted to study nihilanth more. Oh dear. This has soured the deal. The combine no longer want to pay up. Bad for buisness. Gman's employers are all like "Lets reverse this deal. NO MONEY. NO TECH. DON'T WASTE MY MOTHAF***IN' TIME! (Scarface.)". What a hilarious misunderstanding - now Earth is in pieces and it's all for nothing. Except... the combine are so close to taking the human's local teleporter tech - Gman's employers start panicking.
They wake Gordon and set him on a path to assault the Combine and remove them from Earth. Gordon does exactly what the Gman wants him to do - He joins the resistance and continues to spit in the face of the Combine eventually starting a war yardy yardy yar....Think about it. At the end of Episode 2 - With the citadel destroyed, the super-portal closed how will the combine get that knowledge back to the other side? I'll tell you.
B - O - R - E - A - L - I - S
And that my children is a story for another night.