Companies such as that is vitally necessary. If you ever had dealings with a large shit company you would understand (e.g. a company that does not pick up the phones). The relationship between a large company and a consumer is completely asymmetrical.
Most companies can only be encouraged if they receive bad publicity. I do not have experience with GS but I have with a similar site (http://www.hellopeter.co.za ).
Also, it is a free speech issue - if customers of a company wants to voice their complaints about a company it is their right - GS only provides a platform for that.
EDIT: There are some companies (e.g. telephone companies, bad webhosts and banks) whose relationship with the customer is that of apathy or hate. Sites like the above is vitally necessary for that.
Some companies also have a customer service model that is based on crises management and extinguishing fires. To get your problem handled you have to create as big as a fire for them to encourage them to respond.