I was having the same problem--tried to add Opengoo calendar to Google calendar, but couldn't because of a 'can't parse URL' error. On the other hand, downloading the .ics file, then importing into Google, worked. This last experiment actually led to a solution...
Some research on the web (lots of people with the same problem) suggested it had something to do with the http headers, since the calendar content itself was successfully parsed. Looking at examples of what worked and what didn't, it appears the culprit is the Content-DIsposition header. Apparently (and for reasons I don't understand), Google calendar requires that this header be set to 'inline' rather than 'attachment'. When I edited the ical.php layout in opengoo (1.5.3) to use inline and added the calendar to Google it was accepted with no problem.
There is one more part of the story, however. While the opengoo calendar was accepted and opengoo events were immediately shown on the Google calendar, the Google calendar did not reflect any additions I made in OpenGoo. More research suggests that this too is a common problem with Google calendar, but the best explanation I could find is that it is a Google bug (or at least shortcoming). Apparently, Google only refreshes ical feeds infrequently and there is no way for the user to force a refresh. This discussion on the google forums is recent and relevant:
http://www.google.com/support/forum/p/Calendar/thread?tid=7c6b4213a42e6923&hl=enIn my brief experience, the Google calendar did update my OpenGoo changes overnight.
If someone can add to this, it would be great--otherwise the value of adding OpenGoo calendars (or any others) to Google calendar is pretty much wasted since one never knows how far out-of-date an added calendar might be.
Fred