Before you can play the Rally Guanajuato Mexico you need to have come first in the previous 5 rallies (Monte Carlo, Germany, Portugal, Italy and France)
There are 9 gold metals to win in the Rally Guanajuato Mexico, 3 for each stage and each car class:
-SS1 - Guanajuatito ~ WRC, Class 2 and Class 3
-SS2 - Comanjilla ~ WRC, Class 2 and Class 3
-SS3 - Los Mexicanos ~ WRC, Class 2 and Class 3
If you wish you can set the opponent level to 'Easy' and turn off the power ups in the game options menu to make your races a lot easier to win.