Good catching up
- It's good catching up.
- It's a good catching up.
What is the situational difference between the two, and is #2 even correct?
Generally, when #2 is even valid, it refers to a successful pursuit, either literally (eg, a foot race), or in some figurative sense (eg, one sports team belatedly achieving a score equivalent to the opposing team).
On the other hand, #1 generally refers to a discussion where one person finds out what the other has been doing over some past period of time (eg, family affairs, school, job).