The combine forces you fight in HL2/Episodes is like a tiny, tiny fraction of their full force. When they invaded, they probably used some of their real scary shit because earth got it’s ass kicked in only a few hours. You start HL2 like 15-20 years after these events so all thats left now is a few transhuman soldiers and some synths to keep an eye on things.
Thats why you were trying to close the portal in EP2. If it re-opened, the combine would be back and probably packing some of their real toys. Gordon wouldn’t have stood a chance.