First Gold are a hop variety. They tend to be in the 6.5-8.5%aa as far as bitterness is concerned. In order to calculate AAUs you can use the formula:
AAU= weight (oz) x Alpha Acid of the hops.
So lets say you pick up some First Gold hops and you look on the package and it reads 7%.
Using the formula above, .86 oz. x 7= 6.02 AAU. Close enough!!! So almost an ounce of 7% hops will get you 6AAUs.
If you can't find First Gold, opt for Willamette or any of the other Goldings variety. Keep in mind though that the alpha acids in Kent or Styrian Goldings will be lower, therefore you will need more hops to get up to 6 AAUs.
Hope this helps.