Hello 2rlp;
In that case you will need to provide a GSM modem with a Modbus RTU RS-485 connection. Do you have any special model in mind?
The closest item I know of to this are data loggers with GSM modem integrated. I think Sixnet I/O had a model of this kind; I'm sure other manufacturers exist, do a Google on it.
Hope this helps,
Daniel Chartier