G-Man is aware of Breen's plan and tries to prevent the Resonance Cascade, but fails
Here's one of the main reasons, everything stating the words "gman" and "fail" in the same sentence. Pfffft.
Here's my theory:
Harnessing the loyalty of his fellow controllers, convincing the grunts, and enslaving the vortigaunts, Nihilanth becomes the near-absolute ruler of Xen. Your scientist friends find an interesting material which they decide to experiment on, for reason unknwon to man, its a video, deal with it. Unknowingly it is actually a crystal, rock, or whatever from Xen, and the experimentation causes a portal to open between the crystals original planet (in this case Xen) and Earth. Again, how the crystal got there in the first place, I don't know nor do I care. Moving on. Seeing this as an opprtunity to take even more power than he already has, Nihilanth sends vortigaunts and grunts to Earth, wildlife wanders into the portals with them. At first they do well in nearly conquering Earth (go Xen!) but Gordon Freeman gets a giant arsenal and blasts the aliens away and then gibs them with his crowbar, sending headcrab gibs everywhere that are bigger than the headcrab itself. In order to stop the alien forces alltogether, a scientist opens a portal mechanically and lets Gordon into Xen to kill Nihilanth. He succeeds, and my giant, robotic-butted hero dies, freeing the vortigaunts and leaving the rest without a leader. As payment for their freedom, the vortigaunts offer peace and alliance with Earth. Earth has Xen's back, Xen has Earth's. The grunts and controllers, however, decide to leave the whole Earth thing alone and disreguard anything having to do with humans. Nihilanth is reborn (refer to Redemption) but, remebering what happened last time, conceeds to the alliance with Earth. Which explains why you never see them in half Life 2. However, then came the combine. Their power was even greater than Nihilanth's, and they took over Earth and re-enslaved the vortigaunts, not to fight, but to be used as janitors and guine pigs. Some escape and team up with the Lambda resistance group or help Eli with Black Mesa East. Follow the story of Half Life 2 from here.
I realize this may bring up a few questions, but thats my theory, and I'm standing by it. I still don't know who the gman is, but I do not believe he is capible or failure.