Will this bolt directly to my stock catalytic converter?

Yes. This catback system connects at the cat converter outlet — you keep your factory or aftermarket cat in place and bolt the exhaust piping downstream of it.

Does this come with gaskets, O2 sensor bungs, or mounting hardware?

The system assembly is included. Check your vehicle's service manual for gasket specifications and torque specs — those are specific to your year and engine configuration.

Do I need to remove anything else to install this, or is it truly catback only?

Catback only means everything from the cat converter back. Your muffler, resonator, and hangers will need to come off — the new system includes its own mounting points and hardware to secure it in place.