您已经创建了查询，但是没有执行。

You have created the query but you are not executing it.

add_action("gform_after_submission_3", "input_fields", 10, 2);
function input_fields($entry, $form){
    global $wpdb;


                    $entry["1"] = '$name';
                    $entry["2"] = '$email';
                    $entry["3"] = '$purchase_date';
                    $entry["4"] = '$order_number';

   $SQL = "INSERT INTO users ( userID, email, name, purchase_date, order_number) VALUES ( '', '$email' , '$name', '$purchase_date', '$order_number')";
   $wpdb->query($SQL);
   }

此外，我假设您已经创建了 >表。

Also I assume that you have already created the users table.