When you enable Location in your phone, that involves linking your phone to various GPS satellites in the sky. Any online connectivity like WiiFi or cellular are only incidental, to help the GPS triangulation determine where you are located.
Basically GPS signals emitted by those satellites are a different type and frequency than WiFi or cellular signals, so you don't need to worry about a cellular service or its roaming charges to use GPS. GPS is based on its own different type of wireless signal.