Feel, I've looked at all of the diagrams for Leon engines and they all have the coolant temperature gauge sensor (G2) in the same unit as the coolant temperature sender (G62). The VR5 and VR6 engines have this composite sender screwed into the thermostat housing, but all the 1.8t engines (APP, AJQ, ARY, AYQ) have the unit screwed into the coolant manifold attached to the cylinder head.
Now, it won't be the first time that the drawings are wrong, so can you tell me which engine you have and what makes you think that the temperature gauge sender is in the thermostat housing?
One easy way to tell if the unit you are looking at (green or black) contains one or two sensors is to count the number of wires going in to the connector. If it's more than two (could be three or four) then you have two sensors in the block.