Timbo-
GPS pinpoints your location but does not provide any map data. (ie = rivers, lakes, streets, etc.) It is possible for Google maps to "pre-load" a certain limited area using Wi-Fi while you are still in the hotel. Just open the app and scroll around the area a bit. I've done that before. That would be free.
The problem is when you leave the Wi-Fi and then zoom in and out or you walk outside that area. Then you lose map data. You have to either find another Wi-Fi hotspot or turn on your cellular data. That's when the outrageous foreign roaming charges begin.

When using the traditional map apps, cellular data is normally used on the fly as you move around to build the map (street names, etc.) It can get expensive to use roaming cellular data in Europe or Asia for on-line maps like Google Maps, etc.

That is the part you avoid by using CityMaps2Go. The entire map is stored off-line right on your phone. No cellular data needed.

All of them (including CityMaps) use GPS to find your location on the map.
GPS is still free. (at least until the Air Force decides they need revenue)