Can My iOS Device Be a GPS?
Probably. If you have an iPhone or a cellular-capable iPad, then you can start using GPS apps right away. Owners of wifi-only iPads or all iPod Touch models do not have this luxury.
However, external GPS receivers do exist. Some connect via bluetooth, while others can plug into the Lightning port on your device. We have no direct experience with any of these systems, so the best we can do is tell you that our research shows they are out there. You could use one of these to allow your wifi-only device to track your location, or to increase the accuracy of a GPS-enabled iOS device.
There are many apps for GPS navigation to choose from (the AppleVis website lists many of them and describes their accessibility, or lack thereof). Which one you choose is entirely up to you. All of these apps will install whether or not your device is GPS capable, so be sure you can access GPS data before paying for an app.
The other consideration is maps. Most apps rely on a constant data connection, downloading small bits of the map as you move. This keeps the app small, since full GPS maps can be a few gigabytes in size. The downside is that this method needs a data connection, something wifi-only iOS devices can't offer when away from wifi networks. If you are looking for a good GPS app, consider whether you need locally-stored maps.