The bin/gpm index command does not work on a 64-bit Windows 10 machine because it is a 32-bit executable. Dame would be true for the follow-on “bin/gpm install” caommand
That should be " the same would be true for the follow-on “bin/gpm install” "
actually it’s not an executable at all. it’s a PHP file that runs with the PHP executable: https://github.com/getgrav/grav/blob/develop/bin/grav
It’s probably a problem with your PHP installation.
I run Windows 10 64-bit and can run the Grav commands without issue using both 32-bit and 64-bit PHP, and every version between 5.6 and 7.0. Simple as having php.exe available in environment paths and running
php bin/grav clearcache for instance.