Ugh! Why are people hating on Proxy Rack? You simply don't understand the application!
Proxy Rack switches your IP every time the port is opened so for GSA it's not necessarily a good idea because you need some continuity in your proxies for verification, etc. Having a fat list of private proxies from a provider that assigns you a set list and using public proxies that GSA scrapes itself is the way to go.
Proxy Rack is an EPIC tool that works exactly as described and is very useful for the right project.