You can uninstall or removes an installed gfortran-mingw-w64-x86-64 package itself from Debian 7 (Wheezy) through the terminal,
$ sudo apt-get remove gfortran-mingw-w64-x86-64
If you would like to remove gfortran-mingw-w64-x86-64 and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove gfortran-mingw-w64-x86-64
If you use with purge options to gfortran-mingw-w64-x86-64 package all the configuration and dependent packages will be removed.
$ sudo apt-get purge gfortran-mingw-w64-x86-64
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove gfortran-mingw-w64-x86-64