Try and get a good look up the back edge of the motor inboard of that exhaust collector / connector. Your oil might be streaming down from the valve cover gasket, the EGR grommet back behind cyl #7 or the oil pressure sensor on the top left corner behind the intake manifold. All three will flow down generally the same area and produce the same sort of buildup in your picture. Before you put it in the shop for $500+ worth of labor for the pan gasket and find out that's not it.
I detailed the pan gasket change in my 'Hey Vortec Guys' topic, it's easy but a little bit tedious. Have to lower the front differential to drop the pan. IIRC 18mm, 4 bolts for the crossmember, and 22mm for the differential bolts and an impact gun or appropriate breaker bars. Then a bunch of 10/12mm for the pan and attached bracketry. I only went an hour or two over book time, doing our Tahoe in our driveway. Then did the Sub a few weeks later and somehow was slower on that, for no particular reason.