I don't know how to get connected to the ABS in a GT, its a tricky car in general to connect diagnostics equipment without a Fiat Examiner - which 99.9% of dealerships don't even have access to anymore.
It is most likely a wheel speed sensor I should think or just dirty sensor/pickup ring, complete stab in the dark really but thats where I'd start. Its unlikely to be fluid level, as where has the fluid gone, you'd usually notice a soft pedal and poor breaking if there's a leak but take 2 seconds to open the cap and look so check anyway.
Go to each wheel, make sure the pickup ring is not hugely dirty cause it could be that simple, same goes for the sensor itself - there's a magnet in them so it can over time just get ferrous debris stuck to it which interferes with the signals. Following that disconnect the wheel speed sensor and measure the resistance across the sensor. You should get a nice steady reading - don't know what the spec should be but in most instances for one of these sensors you should get between 800 & 2000 ohms. What you don't want to see is no resistance or open circuit or a constantly changing reading. All sensors should return a very similar value.
Can't remember if it has 2 or 3 wire type sensors but if its 2 thats easy, measure between the 2. If theres 3 wires one will be ground, one will be 12v switched live the other the signal. meter on continuity one lead on anything metal on the body as it should all be earthed and check each lead til you get a beep from meter confirming continuity - that one is ground. Then meter on volts DC, again one lead earthed and probe the connections with the other with ignition on and one should have 12v - thats the 12v switched. The remaining will be signal - do the above resistance checks between the ground wire adn signal wire. (it'll be a simple 2 wire now i've written all that out haha)
