In Why would you not accept a free great person? we have a comment, twice stating:
You can also sell a great person for decent money too.
I didn't know about that, where and how?
You can no longer sell units for gold. The commenter on that question is misinformed.
See "Balance Changes" section: https://civilization.com/news/entries/civilization-vi-fall-2016-update-now-live
Deleting a unit no longer provides gold.