“If we are in a location without a cellular signal, then GPS in our phones will not function perfectly. Conclusion: GPS utilizes signal from cell towers, not satellites.”
Some flat-earthers made the above conclusion. What they meant by ‘GPS’ is surely the app Google Maps or another similar app.
GPS is a navigation system that provides location and time information to a GPS receiver anywhere on the Earth, as long as there’s unobstructed line of sight to GPS satellites. The GPS system can only provide these information: longitude, latitude, altitude and time.
Google Maps and other similar apps use GPS in order to determine user location, but the apps themselves also provide a whole lot of other features. The apps can display maps, show routes and e.g. tell you nearby restaurants and gas stations. These features are beyond what GPS itself provides. Google Maps obtains these information from their servers and require an active data connection in order to download the necessary data. That’s the reason why Google Maps doesn’t function fully without a good cell signal reception.
Without cell signal, GPS itself is still very much usable. To verify, you can use an a plain GPS app without all the features provided by a full-blown navigation app like Google Maps. Example for Android: GPS Status & Toolbox, GPS Test.
This ambiguity of the term ‘GPS’ arises because all smartphones today are equipped with GPS. When someone refer to ‘GPS’, they usually mean the app, like Google Maps. But in reality, there are more to the app than just GPS, and GPS itself is not exclusive to smartphones.
There are actually standalone GPS devices which are not in the form of smartphones. Before smartphones exist, practically all GPS devices were standalone.