catalytic converter replacement

My 98 yukon has spit out catalytic converter codes and i've been recommended to change it out. I have a dual split exhaust. With the direct fit models are gaskets installed with these or what?

What codes number one and what gaskets, none that I know of.

the codes were p0420, p0430, and p0446. I Don't know what, if any gaskets. I read a manual or something and it saaid something bout gaskets when replacing the converter.

The codes point to a cat, I don't think there are any gaskets involved. 0446 is an evap leak type code, not cat related.

I had the truck running and i felt some air coming from where it connects to the exhaust manifold. One of the nuts is also missing. I figure this is the cause i have this problem. If i added the other nut and tightened them all up would this problem clear up without me having to change the cats or what?