As best I can tell, Google and Growl use the Google Notifier Growl notification instead of displaying its own.
The GrowlMail plugin lets you configure what info is displayed upon receipt of a new message.
Is there a way to configure what is displayed by the Google Notifier a la GrowlMail--in other words, to choose that I just want to see e.g. that there is a new message and the time, but not the sender or body, etc, etc?