I have been using the Gravity theme for years and now find that I would have a use for both columns and extracontent. I cannot find a users guide (if there ever was one), The Gravity web page talks about columns and extracontent but I do not find any instructions or examples. I could use a stack but the web page says it is built-in.
There is a guide for Extra Content at the Themeflood web site:
I don’t have Gravity, but the other Themeflood theme I have has a checkbox that says “Preview extraContent Containers” – it’s at the end of the master style options.