The first thought that comes to mind is - why not fit a longer wiper blade that wipes across the sensor correctly? I don't know exactly what size the GT requires, but perhaps the driver's side blade needs to be longer than the passenger's, and perhaps someone has fitted two shorter blades the same length instead, or switched the two blades. On my 156, the passenger's side blade wipes across the sensor.
My rain sensor appears to be only half-stuck to the glass and the spring-loaded flap inside the car is also hanging down (it is designed to press the sensor against the glass). Surprisingly my rain sensor is working perfectly, so I'm not going to try to re-attach it!
I discovered a new trick to 'wake up' the rain sensor on each drive: previously I had moved the stalk up to 'off' and back down again. If I had a handbook, I think it would say to do this, to give an 'acquisition stroke' - I remember that in my Stilo's handbook. Do you like an acquisition stroke?
Well, an alternative is simply to pull the stalk and wash the windscreen. The wipers run during the washing routine, then stop, and if it starts raining after that (even, say, 20 minutes later) the wipers spring into life uncommanded.
Rain in NZ and particularly the Waikato has the amazing ability to vary from nothing to downpour in seconds, then back to a few spits, then to steady light rain, and so on. I find the rain sensor especially useful at night, when you don't notice the buildup of a few drops until you get dazzled by oncoming traffic. The rain sensor tends to see the droplets before I do. Works better than the one in my Stilo did, that's for certain.
I think another thing that helps is if you polish the glass over the sensor, so that droplets form individually and the wiper clears them effectively. If the droplets stuck together or were not cleared, the system would probably think it was raining really hard and apply the wipers continuously. On the other hand I imagine anything like Rain-X will confuse the results, as droplets would clear themselves irrespective of wiper speed, so the system would 'learn' not to use the wiper!