Will Glad0s help fight against the Combine?

Sorry, but I doubt it. GLaDOS is trapped in Aperture Science, far from where the fighting of the combine takes place. Mixing the two games wouldn't really work, but I imagine they will often make references to portal, but more than they did in portal 1/the other half-lifes. Also, you're forgetting that you have been in suspension for nine-nine-nine-nine.... With all that destruction, I think the gap in time is pretty great. May or may not be a big time difference, though it looks like we'll just have to see :)