I have downloaded (and installed) Gmail+Growl and it WON'T work at all!! I have the latest version of the Gmail Notifier and am running a fully-updated iBook with OS X 10.4.5.
That's curious - does it install at all? (I've had one guy who's had trouble installing it, and I've identified the issue but have yet to work around it completely.)
This is an obvious question before we can proceed with troubleshooting (similar to "is it plugged in?") - do you have a Growl preference pane in System Preferences? If you click it, does the first tab report that Growl is running?
Don't feel dumb - it's supposed to offer to install it for you, but that feature is apparently broken, and we're investigating it. We snuck a note in under the download link, but you downloaded before it appeared.
Actually it hasn't worked since yesterday, not because the gMail growler is broken but because all gMail notifiers are - Google is said to be "working on it".