Try gmail-indicator. The person who made this did this for 13.04+. From the read me:
INSTALLATION
Make sure IMAP access is enabled in the settings on your gmail
account. Then do:
sudo apt-get install python-pip git gir1.2-appindicator3-0.1
sudo pip install imapclient
git clone http://github.com/bulletmark/gmail-indicator
cd gmail-indicator
make install # Do this as your normal user, NOT as sudo/root.
Edit your gmail username & password in ~/.gmail-indicator-rc. Also
change the command option there to something that works on your system
if you want an audible sound on new emails. Test that command manually
first. Disable, i.e. comment out, the command option if you don't want
new email sound. You can also use the folder command to check another
label/folder, e.g, [Gmail]/Important, rather than the default INBOX.
IMPORTANT: Test the program. Check for reported errors, missing
packages, etc:
~/bin/gmail-indicator -di # to test
(<ctrl-c> to stop application)
Search for, and then start, the gmail-indicator app in Ubuntu or GNOME
It will start automatically at log in and run in the background
checking for new mail. On Ubuntu, you can also start gmail-indicator
from the Ubuntu messaging menu.
REMOVAL
cd gmail-indicator # Source dir, as above
make uninstall # Do this as your normal user, NOT as sudo/root.