Populate Gravity Forms in an ACF select field named "subscribe_form_header"

gf.php

<?php

function populate_gravity_forms($field) {
  if(class_exists('RGFormsModel')) {
    $forms_array = array();

    $forms = RGFormsModel::get_forms(null, 'title');

    foreach($forms as $form) {
      $forms_array[$form->id] = $form->title;
    }

    $field['choices'] = $forms_array;
  }
  return $field;
}
add_filter('acf/load_field/name=subscribe_form_header', 'populate_gravity_forms');