My UPR came with a gasket. It's been on there for a few years and I have had no leaks whatsover.
Only thing I suggest doing to the UPR is coating it with a High Temperature Flat Black paint. I don't have a spec of rust and my cars has been driven in the rain, snow and salt, for the past couple of years with no sign of corrosion. Cost $10 in paint.
I see in your sig that you have flows welded into the stock catback. I have the same thing on my '02. I originally bought a BBK x pipe and with the flows it sounded HORRID. One of the worst sounds from an exhaust that I've ever heard. Bought a Pro Chamber and I'm happy as can be with it. Since you have the flows I'd stick to a Pro Chamber or an H pipe.