How is it Google Maps works without data? I ran out of data for this month so disabled it. Google maps still works in map view when I'm following my route. How is this possible? Is it that GPS can still work without wifi/data and as long as Google Maps has the map it just moves the indicator along?
The GPS sensor itself is a separate entity. It can report your current position with driver software. Google Maps doesn't need a data connection for that.

So where is the map data coming from? Apps cache data. If you've been through the area already, or if the map data has been downloaded, Google Maps will do its best with the map data it has. It won't be able to update you on traffic conditions, as those have to be grabbed in real time, nor will it work for an area for which you don't have cached map data.