Alternatives to Google Notifier
Solution 1:
Just installed Mail Notifr - $2.99 on AppStore, or free if you download the previous version here: http://ashchan.com/projects/gmail-notifr
At least the paid version looks nice, clean, and in terms of functionality it's pretty similar to Google's Notifier, less Calendar notifications. Mail check frequency can be set in Preferences (Google's was a hidden undocumented feature); multiple GMail accounts are supported. I just hope it's stable and reliable enough. We'll see.
Update: so far so good. One little nice thing about this app is that when the system wakes up from sleep it checks for email immediately, whereas with Google Notifier you had to wait another 5 minutes or force mail check manually.
Update 2: one other thing missing compared to Google Notifier is handling of mailto: links and in general, redirecting to GMail when trying to send mail from any desktop app. Mail Notifr doesn't do this. Would be nice to have actually.
Solution 2:
One tool is Notify.app which has long been abandoned by the developer but still works flawlessly (OS X 10.9.4):
https://www.macupdate.com/app/mac/32217/notify-pro
And the free pro license upgrade here:
https://anonfiles.com/file/6bdc702e9f055dcc744b7110a620f1f6
Do note, it uses Growl for the notifications, not the (new) in-built Notification Center in OS X that Apple had added - so you will need to install Growl (which is now a paid app on the Mac App Store and no longer free, though you can probably find older binaries of the formerly free Growl all the same) and then notifications will show.
Solution 3:
If you use Google Chrome, you can try Google Mail Checker (https://chrome.google.com/webstore/detail/google-mail-checker/mihcahmgecmbnbcchbopgniflfhgnkff). Although it doesn't have notifications pop up when you receive new emails, it will tell you how many unread messages are in your inbox.