Could be gearbox oil, check the drive shaft seals (big rubber things on either side of the back of the gearbox with driveshafts coming out of them). They sometimes split and let out the oil which is generally clear. This is the most common cause of gearbox leaks.
Depending on what type of gearbox it is, checking the level is a bit of a job. You need to undo the filler plug (which should be about halfway up the front of the gearbox and is probably a 12mm allen key plug). The oil level should be up to just below this plug.
You can top it up with, preferably, the proper gearbox oil from FIAT or we sometimes us Castrol VMX or similar. The seals are generally cheap to buy but can be tricky to install since you need to remove the driveshaft from the gearbox (generally means removed the driveshaft from the car) take the tripod joint off the end of it, swap seals, put tripod back on, and reinstall into the car.