If you are running a 4 bolt DP there shouldnt really be a need for a gasket, but im not sure about he 5 bolt version. As for the wastegate, you probly wont need one there either, alot of people dont run one because they blow out quite often causing a pretty bad exhaust leak. You can use a little red copper RTV to seal the DP if it leaks, but i dont think it would hold up well on the wastegate.
i use red rtv on manifold to head, turbo to manifold, and 5 bolt j pipe toturbo. all with good luck. i also like to tack weld the four turbo nuts to the manifold studs as they almost always come loose.