How do I revert the default mail client back to Mail from GMail?
This can be disabled in Google Chrome. Go to Settings, then select "Show Advanced Settings..." at the bottom of the page. Click "Content Settings", and click "Manage Handlers" under the "Handlers" heading. Under "Active protocol handlers", change mailto to "None".

Then go into Mail.app and into Preferences -> General, and make sure "Default E-mail Reader" to "Mail"