Tref, your thermostat should have come with the proper rubber sealing ring/gasket that goes around the thermostat. That sealing ring is what stops the coolant from leaking. It should be properly torqued not over or under. The metal thermostat housing does not "touch" to stop coolant, so gasket maker would really do nothing if used. It is possible that the rubber ring became cockeyed during installation. Did your thermostat have a directional arrow on it? The rubber gasket should have slipped around the thermostat, and should have had a protrusion on it with a hole in the middle. It is possible that you were sold a thermostat that was too tall for the housing. I bought mine at the dealer, because the two I looked at at the aftermarket store were both different, no arrow, and too tall.