IF the cat gets real bad it's possible to melt itself or start a fire. But that's pretty rare. Most likely it'll be OK.
Check your warranty booklet in with the owner's manuals & that stuff. I'm pretty sure the catalyst is one of the handful of components that are covered by warranty for 8yr/80k miles. Otherwise, it's worth a phone call...
Some people will probably say it's your 2nd O2 sensor, because that's how the ECU determines whether the cat is working OK. But I've heard that even by 1998 the ECU was pretty good at figuring out the difference between a bad O2 sensor vs. a bad cat.
The gas cap thing is pretty strange. There's a couple completely different codes that would be related to that (P1456 & P1457).