To "get the gist" of something means to understand it a little bit, or to understand the general idea of something. Use it like this:
A: Do you understand?
B: Yeah, I get the gist.
You can "get the gist" of things like:
- the idea of a T.V. show
- how to play a game
- how to do a task at work
It doesn't make sense to say that someone "gets the gist" of a large subject like mathematics or psychology.
If you want to name the topic that a person understands, use "of ___":
He got the gist of how to do it with just a couple of minutes of explanation.