If you want all donations to a specific campaign to be attributed to a specific GAU, you can achieve this by setting up default GAU(s) on the campaign.
On the Related tab of the Campaign, you can click the New button on the GAU Allocation list. This will open a pop-up window where you can choose the default GAU, and then enter either the Amount or the Percent values of the default allocation. Usually you should use the Percent field and enter a value of 100.
You can split the campaign across multiple GAU’s is you wish, but if you use multiple GAUs for a single campaign, make sure the total of the combined GAUS is equal to 100%.