Pre-populate Gravity Form Field Based on Query Parameter

prepopulate-gf.php

<?php

/**
 * Given a URL like http://example.com/?id=1 this function will pre-populate a Gravity Form field called 'fname'.
 * For more info, see GF docs: https://www.gravityhelp.com/documentation/article/gform_field_value_parameter_name/
 */

add_filter( 'gform_field_value_fname', 'populate_fname' );
function populate_fname() {
  $id = $_GET['id'];
  $user = get_user_by( 'id', $id );
  return $user->first_name;
}