Personally, I would not use synthetic in that transmission. Others will disagree. Why not just use the specified fluid and change every 30,000 miles?

You may not need to replace the gasket, but what about the filter? It definitely should be changed.

Much less risky to simply buy the proper transmission service kit.

By the way, if you remove the drain plugs, then you also need new copper crush washers.

Finally, remember that the torque on the pan bolts is very light. 60 inch pounds.

Steve