Setting gmail as default for sending files via email
After you install gnome-gmail or desktop-webmail for Ubuntu 16.04 you need to set it as the default mail application. Some instructions online say to go to Applications - Internet but that is for older versions. For Ubuntu 16.04 you need to go to System Settings -> Details -> Default Applications where you will be greeted with this screen:
The second option Mail will contain these options (after installation steps in the question):
- Thunderbird
- Gnome Gmail
- Desktop Webmail
In the example above Gnome Gmail has been selected as the default mail application. Now when you click to send files in LibreOffice gmail will be used instead of Thunderbird configuration popping up.
If you use Chrome you can navigate to Gmail or Inbox and on the far right side of the omnibox (address bar) you have an Icon with two overlapping diamond-shaped boxes. 
Click this and you get a question if you want to bind mails to Gmail/Inbox. By clicking "Allow" Gmail/Inbox will be set as your default mail program.