Do I need new gaskets or clamps to install this?

You'll want a gasket between the catback and your catalytic converter flange — the assembly itself doesn't include one. Grab a universal exhaust gasket kit and stainless bolts from any parts store. Everything else bolts straight to factory points.

Will this work with my stock catalytic converter?

Yes, the catback bolts directly to whatever cat you currently have. If your cat is clogged or failing, that's a separate fix — this system doesn't address anything upstream of the bolt-on point.

What about sound? Will it be loud?

That depends entirely on your engine setup and any resonator you're running. A catback alone won't drastically change tone — it's the muffler design and what's upstream that controls volume. Test-fit and listen if possible before final install.