You said you tried using a different zip code. Did you also try entering your own zip code, and looking for a new lineup? Often during a transition like this, there will be a lineup labeled "Comcast YourTown (Rebuild)".
I think Adam simply made the mistake of thinking that the question was about using Google fiber for the Internet connection to download the guide data, and didn't realize they were also a cable TV provider. I admit that when I first saw the question this is also what I assumed.
But as long as Google provides a converter box that ReplayTV can control with the IR Blaster, and the lineup is available in the guide service, it should work similar to cable or satellite TV....
Are you saying that the ReplayTV is connected via a network to a router, which is connected to a dialup Internet modem?
On the ReplayTV, go to Setup -> Network & Input Settings -> Change Network -> Change from Automated to Manual.
Then you should be able to enter all the IPs by hand.
Suddenly tonight my Comcast Motorola DCT2000 cable box started doing the old "triple digit" thing with the IR Blaster -- the blaster sends "4", but the box sees "444". I've been using code 4476 for years without this problem (but occasional dropped digits).
I just switched to code 0476, it looks like it's better. The strange thing was that this was the code that had the problem years ago (there are several locked threads from 2003 and 2004 about it).