When the portal is destroyed, do you think anything happens on the other side? The Earth end explodes, but does it explode on the Combine end?
If it does, then it stands to reason the G-Man knew it would, and that was part of the reason why he manoeuvered you into doing it.
If it does, then it stands to reason the G-Man knew it would, and that was part of the reason why he manoeuvered you into doing it.