You know that DP isn't open neck? Probably academic as you already have it, but, nevertheless...

As for the gaskets, the one on the left is the up-pipe to turbo one. You won't be disturbing that if all you're doing is replacing the DP. The one on the right is turbo to downpipe, and you really should fit a new one, but you may well get away with re-using what's there at the moment, provided the mating face of the new downpipe is well finished.

You may also need the donut that goes between the DP and centre section. All depends on what the design of that end of the downpipe is like, and whether it comes with its own replacement gasket.