The GSG is a specialty group within the Association of American Geographers. You must be a member of the AAG to be a member of GSG.
To join the AAG, visit the AAG membership page. From there you can request the online form to join. The cost of membership is graduated based on income. There are also special student rates.
Dues for the GSG are in addition to AAG dues. Regular dues for the GSG are $10 per year and dues for students are $0 per year.