You need a polish ip address
Make an account at http://www.cdaction.pl/
and then go here: http://www.cdaction.pl/seriale/GT_Legends
The code will be emailed to you. Enter it into Steam without dashes.
Quickie Proxy Directions
1. Google "Free Proxy List" or something like that and find a Polish Proxy. It should be in the IP Address:Port Number
2. In Firefox, go to options - advanced - network - settings
3. Check Manual Proxy Configuration, enter the IP address in the first box and the port number in the little box and check Use this Proxy Server for all Protocols. You have now enabled a Polish Proxy
4. After you're finished, undo all these steps to disable the proxy in Firefox
Alternatively, you can also use an in-browser proxy (Like http://webprox.pl/
(thanks OddFull Metal)), but your mileage may vary on those.