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

The GSA Reference Guidelines are attached in the link below. It does not give specific guidance on citing indirect sources, however, you will notice that the last bullet indicates "For references that do not match any of the examples given here, include all information that would help a reader locate the reference."  If your school has a Writing Center, I would encourage you to run this by them, as they should be familiar with what GSA citation rules are common at your institution.