Who replaced the head gasket? If it was someone like me, I would not feel terribly surprised if it failed 20,000 miles later. If the engine is now burning coolant and\or or oil I could see that damaging the CC and the unit itself can become damaged if it overheats.
However, this guy's Chrysler's engine blew the gasket when the cat clogged:
https://mechanics.stackexchange.com/...he-cat-to-plug