G-Man is an interdimensional being of almost godly proportions.
His gateway through dimensions ( Xen ) is taken over by a rogue Combine creation, the Nihilanth.
Being stranded in our dimension, he uses what resources are available to coax the Nihilanth into our dimension in order for the Combine to come and destroy it.
Unbeknownst to him however is Gordon Freeman. In destroying the Nihilanth, earths teleportation knowledge is up for grabs ( it would surely of been destroyed otherwise ). The Combine wish to use this knowledge to take control of Xen, the interdimensional gateway, their whole purpose for creating the Nihilanth in the first place.
The G-Man, fearing this possibility, turns to Gordon Freeman to stop the Combine, whatever the cost.