Lets look at the chain of events, black mesa incident opens a rift for the Xen, Gordon defeats the Xen.
Opposing Fronts, Sheppard defeats Race-X (another race competing with the Xen for earth).
In the power vacuum Combine take over, Gordon significantly damages the combine efforts.
Three different powerful intergalactic races all defeated/destroyed/crippled.
I'd assume hes from a fourth, potentially more powerful civilization is taking down the others. But maybe like the ancients from Stargate they can't interfere obviously. So the Gman, representing them, manipulates Gordon (and Adrian) to achieve their goals.