The OP's question may sound dumb but it is a good question as most aftermarket exhaust components are a different size than the stock system. I'm also in the market for a new dp to catback gasket. The one that came with my Helix dp just went last week.
Now that I think about it I also need two gaskets. The first from the dp to midpipe and the second from the midpipe to the axleback.
The midpipe to axleback is holding up pretty good via plenty of gasket maker but I would have rather of done it right. Well see if just gasket maker and some stronger bolts will do a better job on the dp to midpipe gasket.