BU stands for Bitterness Units and GU stands for Gravity Units.
Ray Daniels, in his book Designing Great Beers breaks this ratio down to a very simple Bitterness Units to Gravity Units ratio, commonly noted as BU:GU. The Bitterness Units are just the calculated IBUs of the beer. The Gravity Units are what you get when you subtract one from the Original Gravity and then multiply by 1000.
By dividing your bitterness units by your gravity units you arrive at a ratio that lets you know how bitter your beer is relative to the 'sweetness of the malt'.
A light American lager might have a ratio of 0.29 whereas an Imperial IPA might have a ratio of ~ 1.13.
BU/GU is a good indication of bitterness but water chemistry, yeast selection and how you use your hops will all affect ‘perceived bitterness’.
However, the recipe creator uses this information along with the BJCP style guidelines to show if your recipe is within specifications for that style.