The best way to earn Gold is to attack friends. You can only do this once a day for each friend so to get more Gold you need to invite friends. This can be done by doing the following three steps.

Step 1
Sign up for an account at Z2Live.

Step 2
Login with this account, then invite friends.

Step 3
After the friends accept your invitation, they are in your friends list. As they play Battle of Nations, you can attack them.