For some reason GCal.app has stopped functioning for me. (I'm using MacOS 10.5.2) Google Calendar no longer appears...just a white page. There must be some change in the way Google can be accessed...the same thing happened when I tried to use Hana as well.
A good solution I found to be Mozilla Prism (http://wiki.mozilla.org/WebRunner )
The download can be found 2/3 down the page.
This app is still under development, but great to use already... it can create shortcuts for any web-application in your dock with any icon, which will act just like a normal application. You can choose whether you or not you want to see the navigation bar, se status messages or enable navigation keys. For me this is so far the best I found regarding web-applications...and for free!

I really like this and use it, but one thing I'd like to see is Gcal remember it's window position between closing and opening the app.

a fair app but to be honest why limit it to just calendar? having mail as an option in the prefences would work wonders

another idea would be to use a firefox base rather than webkit so you could have the messenger running too, but i know nothing about that so ignore me if thats a crap idea