A new site, recently launched, called LevelMy360.com offers a way for people to increase their Gamerscore.
The service basically asks for your Xbox Live account information. They play games on that account gaining you Gamerscore.
Prices start at $39.99 for 500 points all the way up to $299.99 for 3000 points. This is not a Microsoft sanctioned service, and so buyer beware, mind you, you would have to be extremely desperate to need to pay to get Gamerscore.