Thank You, adding PHP via custom plugin does work. For the others:
1. Instal Pluginception:
2. Create Plugin (suggestet name NextGEN and Generatepress Integration)
3. Put the code:
<?php
add_action( 'after_setup_theme', 'tu_move_slideout_navigation' );
function tu_move_slideout_navigation() {
remove_action( 'wp_footer', 'generate_slideout_navigation', 0 );
add_action( 'generate_before_footer', 'generate_slideout_navigation' );
}
?>
4. Save new plugin
5. Activate new plugin
6. Check if everything works