Since the oil leak is new and you just replaced the valve cover gasket, the first suspect would be the new gasket is not seating correctly. There are a number of other sources for a oil leak in that area including: oil pan, crank rear main seal, crank rear main cap, oil pressure sender, head gasket, transmission seal. If the source is not obvious, I use one of those aerosol can engine degreasers that you hose off. After the engine dries, start it and look for any new wet areas.