Q. How do you cite an encyclopedia in GSA format?

GSA does not provide examples or instructions on many specific formats. They say that if you encounter a format for which they do not provide an exact example, use the information and do your best to provide a thorough citation.  In the case of an encyclopedia, you would cite it like a book chapter.

In the first link, Citing References in Scientific Papers, look for the example For Articles or Chapters with separate authors from a Book or Compilation.

If there is no author, the article or chapter title will be your first entry. Be sure to check and see if there is an editor or editors. If so, include that information in the correct place.

I also included the link to GSA's site.