Some GSM modems communicates with an UART. And some of them are RS232, while others are 0-3.3V or 0-5V or whatever. Some other modems uses USB. Some may use SPI.

GSM modems can often be very expensive, when supplied as stand-alone products in their own boxes. And they may be very inexpensive if supplied as a module for mounting on a PCB. But GSM modules can often be quite hard to use because of fine-pitch connectors, and very special needs for the supply voltage.

Receiving vehicle coordinates will of course require a GPS. But not all GPS have a serial port. Some have USB, SPI or other interface. Some supply power to an active antenna. Some don't. And a GPS module may use a different supply voltage than the microcontroller, which may require extra glue logic to let the two interface with each other. Next thing is that a GPS can take a very long time to get a fix, so they normally needs a battery backup to let it remember the location it was, and satellite position info from when power was cut.

It can often be meaningful to use different GSM solutions for the embedded side and for the PC side. GSM on the receiver side is of course only applicable for small-scale tests. I have tested what happens when a large-scale telephony system issues 100+ SMS/second targeting a single mobile phone ;)