Please have a look on the dictionary definitions on the nouns "gist" and "purport" and let me know how they differ in meaning?
The most important pieces of information about something, or general information without details.
The general meaning of someone's words or actions.
It strikes me as if they are interchangeable in most cases. Here I have tried to bring up two examples in which I think using either of then will not make any change in the whole meaning:
- I don't remember his exact words, but I can tell the the ....... of his words.
- I didn't read it all, but the ....... of the letter was that he won't return.
I think we can use these two interchangeably in either case above.