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.