It's not a gasket,but O Rings. To stop the leak, you will need to remove all three pumps. The two you can see, and the one you don't see. The leak you have is is oil getting past the O ring on the back side of the transmission pump, which is within the case just ahead of the gear pump flange. To get to it is a sloppy job but really difficult. Just ahead of the pump stack is a steel plate bolted to the bottom of the main case with eight 3/8" bolts. When you remove this plate, you will get most of the 7 or so gallons of transmission fluid so a large pan is in order here. Once the plate is off, reach into the opening and remove the suction screen from the gerotor pump(it's only held by the friction of and O Ring and pulls off easily). Next put a pan under the left side differential filter and remove the outer filter cover. This can be a mess. There is a springloaded valve inside the can that should stop the rear housing hydraulic oil from entering the can once the filter is removed. You should get a quart or so of oil, and the flow should trickle to a stop, but if the valve plate bolt and nut have come apart over time(can happen from time to time) you get about 15 more gallons of oil in your pan in just a few seconds. The filter cover will require a good jerk to pull it out of the bore, so hang onto it and pull it out an inch or so to let the valve seat. If the oil flow doesn't slow and stop after a few seconds, slam the cover back in and prepare to drain the housing by removing the drain plugs. Next, remove all the line from the pumps and remove the piston pump, then the gear pump. The gerotor pump is bolted to the gear pump from the front side and comes out with it, along with the O Rings causing your leak.