$ sudo apt-get update
$ sudo apt-get install gcc-mingw-w64-x86-64 

gcc-mingw-w64-x86-64 is installed in your system.

Make ensure the gcc-mingw-w64-x86-64 package were installed using the commands given below,

$ sudo dpkg-query -l | grep gcc-mingw-w64-x86-64 *

You will get with gcc-mingw-w64-x86-64 package name, version, architecture and description in a table

gcc-mingw-w64-x86-64 package basic information: