If you changed something on the car side that included that the Gateway was completely disconnected from power, for example you replaced the car battery, then the Gateway might loose its memory. If it lost its memory, than it means it lost the car configuration (for example BMW). Or in rare cases the reason is unknown, the Gateway 300 simply have lost the car configuration, and this is why it worked before for years, but now it is not recognised by the car.
To solve this you need to download the GW_EES.BIN file from the Gateway device. If you check that file based on section A, then you will see that which base software is on your Gateway 300 device (G3AA, G3AB, G3AD, G3AE, G3AF). Based on the base software you can install the matching car config file, and after that the Gateway will work again as before. From below you can download the car config files, installation instructions are included: