Leaking PS gear box at top-need help

Its a return valve. They have a tendency to clog up and this could cause a leak from too much pressure. Unbolt the nipple and clean the internals with some solvent or brake cleaner, make sure it goes back together the same way it came apart! Get some new O rings and run it. No Gasket.

Also check for pitting on the spool valve where the O rings attach.