Potentially dangerous tags are stripped from GD fields.
To allow iframes and other tags in the main post_content Description field use this snippet
/**
 * Skip sanitize specific field values.
 *
 * Skipping sanitize values may result in cross-site scripting (XSS) attack.
 */
function gd_snippet_custom_skip_sanitize_field( $value, $gd_post, $custom_field, $post_id, $post, $update ) {
    // Fields to skip strip tags.
    $skip_strip_tags = array();
    $skip_strip_tags[] = 'post_content';
    if ( in_array( $custom_field->htmlvar_name, $skip_strip_tags ) ) {
        $value = $gd_post[ $custom_field->htmlvar_name ];
        if ( isset( $custom_field->data_type ) && ( $custom_field->data_type == 'DECIMAL' || $custom_field->data_type == 'INT' ) && $value === '' ) {
            $value = null;
        }
    }
    return $value;
}
Also see: https://wpgeodirectory.com/support/topic/script-tags-in-custom-field-not-working-after-update/#post-511899