The original project worked OK, but had the following problems
The A6 GSM/GPRS module and the Arduino Nano were not well-matched because of the need to use a baud rate of 115200. In theory, this could be handled by the hardware serial port of the Nano, but the hardware port of a Nano is hard-wired to a USB interface chip. A lot of messing with interrupts and low-level code was needed to work around this problem.
The physical wiring interface was designed to suit a particular gate controller which expected open-collector NPN. This required several resistors, capacitors, and transistors just to suit this specific interface.
The plan is to create a more generic solution which uses
A GSM module (maybe something easier to use than the A6 GSM/GPRS module)
An Arduino Pro Mini (because the hardware serial port is available)