Rated 3 out of 5 stars
Doesn't seem to work with all Firefox notifications. E.g. the notifications with the extension 'HTML Notifications' show the native notifications.This review is for a previous version of the add-on (1.5.1-signed).
GNotifier works fine with *standard* html5 notifications. Unfortunately Gmail is using deprecated *legacy* webkit API. Try to disable 'HTML Notifications' and install following greasemonkey script: https://userscripts.org/scripts/show/172570. This script rewrites Gmail API to standard one. After that Gnotifier should work! If not, please post issue on https://github.com/mkiol/GNotifier/issues with more details.