If your desktop account is the same as your phone sign on account (the one you entered when you first set up your phone), just go to Settings, My Accounts (or Accounts & sync), click on your GMail account and make sure Sync Calendar is checked.
If your desktop calendar account is different from the phone account just go into Accounts and click Add Account at the bottom of the list page and add the new GMail account as a secondary account. Once it's set up, click on it and make sure calendar is syncing.