Gist means that you understand but only in a basic way. Lets say you start watching baseball. It has many rules. You can't undestand everthing about the game, the rules, the nuances of playing styles, how to pitch the perfect fast ball, or all about it in one day. But you can understand the basic rules in just a few minutes. Once you've done that you've got the gist of the game.

"Gist' means the general idea being discussed or explained - perhaps, it is similar to 'a summary' - all the key points strung together into a short narrative. "Key point", is a single, key message that is central to the whole argument. We might also refer to it as 'the take home message' - meaning if you only pay attention to and remember one thing from the lecture, let it be this - that is the key point. The gist would be the general narrative which puts the key point in context (perhaps explaining why it is so important, how it came to be that way, and what the consequences are if that key point is ignored).