NEVER rely on dash coolant temperature gauge to give an exact reading. They are purposely designed to have a 'dead zone' at the 90deg (or whatever 'normal' may be) mark. It obvs varies by model, but it is not uncommon for a gauge to read 90 - but the actual coolant temperature can be between 75 to 105 deg C.
The GTI (like virtually all cars now) use a specific 'Coolant Control Module' ECU - this is what feeds the dash temperature gauge, and also what powers the cooling fans. There are two coolant temperature sensors, and both need to be working correctly - one on the radiator, and one on either the cylinder head or thermostat housing. Oddly, actual thermostats are generally the most reliable component in the cooling system.