Your best bet is to use this function:
add_action('admin_head','generate_custom_admin_head');
function generate_custom_admin_head() {
?>
<style>
#menu-appearance {display:none;}
</style>
<?php
}
That will hide the “Appearance” section of the Dashboard, which includes Customize and GeneratePress.
A more thorough version which you can use with the above is here: https://gist.github.com/generatepress/f04d0881f806e84be0ac
It will fully remove everything GP related – good for white labeling.
These functions can go in your child theme’s functions.php file, or in a new plugin (recommended) – use Pluginception to create the plugin super easily.