Hmm, I’m not sure about this, but maybe adding a function like this would work..
if ( !is_super_admin() ) {
add_action( 'admin_menu', 'remove_customize_page' );
add_action('after_setup_theme', 'generate_remove_admin_page');
}
function remove_customize_page(){
global $submenu;
unset($submenu['themes.php'][6]); // remove customize link
}
function generate_remove_admin_page() {
remove_action('after_setup_theme', 'generate_create_menu');
}
You want to add this using a plugin like: https://wordpress.org/plugins/code-snippets/
What this should do, in theory, is remove the Customize link and GP menu from the admin area if the user isn’t a network admin.
Untested, so let me know how it goes.