Unless you have some concession with your wireless provider, maintaining all those GSM network devices is going to cost a small fortune in fees. If you don't need real-time monitoring, what I'd recommend is to use meters with Zigbee or some other similar rf instrumentation network protocol and concatenate the data at a neighborhood gateway, where the data is then uplinked to the host site. You could use GSM there, if you want.
We do something similar in one of our products, using a 900MHz radio to uplink telemetry data to a gateway device. The gateway can connect to the servers using either CDMA or GSM radios; but, it has an ethernet port if we want to hardwire it to a server. With the right radio, you could conceivably collect data from several square kilometers using one well placed gateway.