Try these steps:
  1. Close any open Gmail tabs.
  2. Type chrome://settings/handlers into your address bar and press enter.
  3. Remove any entries shown here.
  4. Make sure "Allow sites to ask to become default handlers for protocols" is enabled.
  5. Close this tab.
  6. Launch Gmail and you should now see a diamond icon at the right of the address bar. Click that to set Gmail as your default email handler.
If you don't see the diamond icon to the right if the address bar, you may need to quit and relaunch Chrome for the change to take effect.