Hello,
1. GPS functionality on RUT955 is completely independent. SMS messages or internet connectivity is not necessary (and are not used) for device to acquire GPS information.
If, however, you are interested in not only acquiring GPS position on RUT955, but also sending it to your GPS server, then internet connectivity is necessary.
2. Everything depends on how often you want GPS records o be captured/sent to the server. E.g. collecting information every second, or every minute would have drastically different data consumption. For most relevant information, simply configure router's GPS service to your needs with desired data sending frequency and check how much traffic is generated in one hour.
3. Router only has one functionality, which could consume small amount of data (but not 20 MB in short amount of time) - RMS. It can be enabled/disabled from "System -> Administration -> RMS" menu.
Apart from this functionality, router does not consume any data at all (excluding the fact that maintaining mobile connection also uses small amount of data). If you suddenly see higher data consumption, this means that one of your devices, which is connected to the router, started to download something (or e.g browse the internet). That might not be intentional download, since nowadays most programs "auto update" in the background without notifying the user/asking for permission.
If you remote LTE antennas, GPS functionality will be working as before (assuming that GPS antenna will remain connected), but router might not be able to send this GPS information to your GPS server.