The GSA is now only made for the 17 inch rims. If you want to be able to run the GSA you will have to buy the 17 inch rims (a very expensive choice). Here is a link to an article about the tires available for the HUMMER (this site also has TONS of great info about maintaning an H1).
http://www.amghummer.com/features/Tires/tires.htm