Q. How do I cite an indirect source in GSA style?

GSA Style does not make any specifications about citing indirect sources.  However, it is based on Chicago style, which does have rules for indirect sources.  Chicago style mentions the indirect source in footnotes, but since GSA style does not use footnotes that option is not available.  Based on this, a good approach would be to mention the indirect source, as Chicago does, but incorporate it instead into the in-text citation.

For example, if you have a paper in hand written by Smith in 2006, which cites information from a work by Jones, you could mention both like this:

Jones has suggested that there are more volcanoes in the Pacific than originally believed (quoted in Smith, 2006).

You would then list the full citation for Smith on your References Cited page.

Please keep in mind that this solution is not specified anywhere in GSA Style rules.  But it would enable readers to easily determine where you are retrieving the information in your paper.