Older version of gfortran doesn't get installed despite removal newer version
You can use apps such as App Cleaner to fully uninstall apps. 

In your case there are likely files left on your computer for v6.3.0 and using App Cleaner will remove these additional files.

You can do this manually by finding and deleting the .plist and .bom files associated with gfortran. Type 'locate gfortran' in a terminal window will find the path names to these files.