There is no gasket (usually -- a few non OE ones come with gaskets, but..........) just a sort of polyurethane gasket goo.
Likely casuses are the water pump seal or the seal from the water pump to the water rail (sort of on top of the oil filter). The second one is cheap, if a bit awkward to get to.
Often, if you park the car with the engine running, offside wheel on the curb, you can get your head in position to see which it is.