Being able to capture the amount of traffic being used on an interface would be very useful, especially in a situation where the interface is on a network which incurs cost (like GSM). A simple counter for bytes in/out, reported by the interface, much like 'netstat' shows, would be suitable. Presumably the count could be made in GSM3SoftSerial::finalWrite()?

This would allow an application using GSM data to keep track of the data it is using, and perhaps make adjustments. (Just like a mobile phone has a report showing you how much data you are using).