VSS, GND and -V all mean different things in general, but in a given context, all those different things can be the same
VSS means the negative supply voltage to a circuit made out of field-effect transistors (as most logic ICs are). The S is for Source, VDD is the corresponding positive supply pin (D for Drain): https://en.wikipedia.org/wiki/IC_power-supply_pin
Ground, of course, is the reference voltage that all others are measured against: the 0v line.
-V or V- is the most negative supply voltage.
So for most simple circuits, we don’t have any negative voltages, so ground is the most negative voltage; and unless we have split analogue and digital supplies, we’ll also use that as VSS
On the other hand, amplifier circuits often have split power rails: +V and -V on either side of a GND/0V line. In which case, either GND or -V might be used as VSS for any associated digital circuits (DAC?), as convenient…