Hi!
According to Google's GData documentation, you need to authenticate before you have access to the content inside any of Google's applications. You can do this authentication either via a login for standalone applications, or either via a web based authentication. This is why applications such as the Gmail notifier require you to login first before delivering content. You can read more about google's authentication schemes here:
http://code.google.com/apis/gdata/auth.html
In either of the two schemes, you will only need to login once until you directly demand the application to terminate the transactions, and thus your authorization token will no longer be valid, or until the time expiration period of your token elapses. I hope this information was useful.
/C閟ar