† Google Notifier is Google's application for notifying you on new Gmail messages or upcoming Google Calendar events.
‡ Growl is a way of customizing the notifications to taste.
Previously known as Google+Growl and Gmail+Growl.
First quit Google Notifier from its menu. Then remove the file GoogleGrowl.plugin from the folder located at your home folder → Library → Application Support → Google Notifier.
Growler for Google Notifier makes Google Notifier send out Growl notifications, but for a number of reasons, it is unable to automatically disable Google Notifier's own notifications.
Google Notifier on its own is capable of show one kind of notification that acts and looks in one way. If you'd like for this notification to look differently, you have to make Google Notifier show another kind of notification that looks the way you want it to.
Growl is a way for notifications to look and feel the way you want them to. Growler for Google Notifier is a plugin for Google Notifier, and when installed works inside of Google Notifier to make it show Growl notifications as well. Using the Growl preferences and the many available "display styles", you can then choose a look that suits you better. As an extra bonus, you can configure Growler for Google Notifier to make the contents of the notification different as well.
Growl is available as a preference pane for Mac OS X versions earlier than 10.7 "Lion", and as a menu bar extra sold from the Mac App Store for Mac OS X 10.7 "Lion".