The G Man makes it abundantly clear that he represents one of the innumerable powers in the universe with a strategic interest in planets. He "recruits" agents like Gordon, presumably because bringing people and resources through portals is hard. That's why the Combine use mutated humans, local but upgraded small arms technology, and living weapons that they can breed on Earth. That whole screwy centrifuge thing at the end of HL2 was ONE Combine portal device, just like how it took a building-sized generator for Black Mesa's first attempt; in spite of the overall inferiority of human science in the cosmos human portal technology is decades ahead of other worlds, at least partly as a result of the G Man's tampering. The Aperture portal is, in game lore, actually the most advanced and compact portal device in the known universe, limited only by firing a projectile that travels at relativistic speeds to create its portals (in portal 2 the time it takes between
and the corresponding portal in the lab opening is accurate to the portal traveling at the speed of light).
It's the advanced state of human portal tech that made them a logical choice for the G Man to manipulate into invading Xen, particularly since the encounter suit (which may or may not also be his work) gives humans comparable combat capabilities to the Vortigaunts and their kin. It's unclear whether the subsequent subjugation of Xen and Earth by the Combine is part of some long game on the G Man's part, or whether the Combine's spread was an unforseen consequence (ha) that he is simply making Gordon fix.
So apart from being a crazy alien dude with teleport powers, the only really mysterious thing about the G Man is his long-term plan.
