gravity form upload file to media library and use attachment ID in custom field

save_upload_field_to_custom_field.php

add_action( 'gform_after_create_post', 'gf_add_to_media_library', 10, 3 );

/**
 * Save file upload fields under custom post field to the library
 *
 * @param    $post_id  The post identifier
 * @param    $entry     The entry
 * @param    $form      The form
 */
function gf_add_to_media_library ( $post_id, $entry, $form ) {
  foreach($form['fields'] as $field){

  //get media upload dir
    $uploads = wp_upload_dir();     
    $uploads_dir = $uploads['path'];      
    $uploads_url = $uploads['url'];

  //if its a custom field with input type file upload. 
  if( $field['type'] == 'post_custom_field' && $field['inputType'] == 'fileupload'){
    $entry_id = $field['id'];
    $files = rgar ( $entry, $entry_id );
    $custom_field = $field['postCustomFieldName']; //custom field key

  //if file field is not empty or not []
    if ( $files !== '' && $files !== "[]"){

      $patterns = ['[', ']', '"']; //get rid of crap
      $file_entry = str_replace($patterns, '', $files);
      $files = explode ( ',', $file_entry  );

        foreach ($files as $file) {
          //each file is a url
          //get the filename from end of url in match[1]
            $filename = pathinfo($file, PATHINFO_FILENAME);
            //add to media library
            //WordPress API for image uploads.
            include_once( ABSPATH . 'wp-admin/includes/image.php' );
            include_once( ABSPATH . 'wp-admin/includes/file.php' );
            include_once( ABSPATH . 'wp-admin/includes/media.php' );
           
            $new_url = stripslashes($file);
            $result = media_sideload_image( $new_url, $post_id, $filename, 'src');
            //saving the image to field or thumbnail
            
            if( strpos($field['cssClass'], 'thumb') === false  ){
              $attachment_ids[] = (int)  get_attachment_id_from_src($result);
            }
            else{
              set_post_thumbnail($post_id, (int)  get_attachment_id_from_src($result) );
            }
             
        } //end foreach file
        if ( isset( $attachment_ids ) ){
          update_post_meta ($post_id, $custom_field, $attachment_ids);
        }
      } //end if files not empty
    } //end if custom field of uploadfile
  } 
} //end for each form field


function get_attachment_id_from_src($image_src) {

  global $wpdb;
  $query = "SELECT ID FROM {$wpdb->posts} WHERE guid='$image_src'";
  $id = $wpdb->get_var($query);
  return $id;
}

I copied the code to my functions.php file and changed the "postCustomFieldName" to my custom field name "test_image_upload". but it is not working. when I upload an image to the field, there is no image uploaded to the media library. Any one knows why?

I did this too, but turns out postCustomFieldName needs to stay unchanged. It's the field key that is connected to your custom field name.