The gt40 has Ethernet (RJ 45 onboard) and it also has GPRS for IP traffic with appropriate data SIM and GSM for SMS. The gt20 has Ethernet only. The modules were a collaboration between Chiron and Honeywell.
I have a GT40 on the shelf for development purposes. I wouldn't advise buying one as they're not great. A better option is the IP module - either Selfmon vmod or Honeywell V4.16 module and buy a cheap 4G router to provide a backup IP path in the event broadband goes down. In fact, unbreakable broadband and a UPS on the router is a good option.