I figured it out, the port 80 got me on the right track. This was the issue:
I use Firefox as my web browser, but I used to use IE. When I was using IE, I had a web proxy I was going through, but that web proxy doesn’t exist anymore. Firefox isn’t configured to use the proxy, so web access worked fine, but I’d never bothered to switch IE back to direct connection.
GSB must have been looking at the IE proxy settings for its Internet access, because once I set IE to direct connection, GSB had no issue at all.
Thanks all for your help.