You can pre-populate a field with gform_field_value_$parameter_name
hook, where $parameter_name
is the name of the field/parameter. So, for example if you added that to your code:
add_filter('gform_field_value_invoice_numb', 'my_func');
function my_func( $value ) {
return do_shortcode('[ABC_form_number length="13" char="num" case="false"]');
}
(Note: Allow field to be populated dynamically
option needs to be enabled)