The last thing I would think of using roaming data for is GPS. Navigator map updates (about twice a month) can run into the Gigabyte level. At least they ask you if you want the update now or wait for wifi to be available. (1 GB @ $10/MB makes Koodo a very rich wrestler indeed!).
That having been said, once you have the maps loaded onto your phone, Navigator GPS application uses no data, just the computer and your pre-loaded maps.
Others, such as google maps, will load maps on the fly and incur data charges as you roam. Investigate whatever you are planning to do thoroughly.
And please make sure to turn mobile data OFF unless you really need it.