GSA Approved Shredders are better known as level 6 high security shredders. They are also referred to as NSA Approved Shredders or DOD Approved Shredders.
About GSA
“GSA provides workplaces by constructing, managing, and preserving government buildings and by leasing and managing commercial real estate. GSA’s acquisition solutions offer private sector professional services, equipment, supplies, telecommunications, and information technology to government organizations and the military. GSA policies promote management best practices and efficient government operations.”
A few well recommended GSA Approved Shredders or Level 6 Shredders: 
SEM 266/4 High Security Shredder
SEM 244/4 High Security Shredder
Kobra 400 HS6 High Security Shredder
Full Listing of HS6 GSA Approved Shredders
