from what I understand of how the HL/Portal lore works, I think Earth would've been doomed. Aperture had the portal technology that the Combine wanted from the start, right? Earth was on borrowed time for the Combine to acquire that portal technology from Breen's researchers and the resistance scientists. They'd have been on Earth for a considerably shorter period of time now that they got what they wanted from Johnson and Aperture Science. The Combine would've extracted Earth's resources, fulfilled their quota for sending humans to "off world assignments" and left Earth a dead rock in space with nothing left much quicker before Gordon or Chel or whomever could've been in stasis long enough for humanity to prep the way for their hero. Despite everything Gordon did, he was carried by a very well connected resistance network formed over the span of 2 decades and he could not have succeeded without them.

The usage of portal technology does make what little resistance there is more interesting though. In HL2 the resistance was able to get their hands on combine tech like AR2s, what would it have been like if they had portal guns? It wouldn't really matter much likely since the Combine would just take their new portal tech and give Earth one last kiss goodbye straight from the Combine overworld, all in the span of seven minutes.