Many GMSL parts have a line monitoring function which can be used to check for problems with the serializer -> deserializer cable, such as: short to ground, short to battery, or open circuit cable. Some customers may want to do their own line monitoring using external ADC's to add specific functionality.
The line monitor circuit does not interact with any other function of the parts, except that it causes the /ERR pin to go low, and even this can be disabled; so you can use an external circuit. Note that the line monitor pin on GMSL parts has robust diodes that will clamp the voltage at approximately -1V and +4V, you can still use this capability, even if you do not use the line monitor function of the part. When adding an external monitor circuit, use resistors of >5K connected to the coax or STP line, so that the circuit does not affect data integrity on the 50ohm or 100ohm cable, and the current is low enough for the diodes to provide protection.