Using it before the buffs and using it after the buffs results in an average potency difference of 200 over 90 seconds in favor of Gauss Barrel being used during the opener, it's very minor actually. This difference comes from the extra auto attacks you can get in before casting Gauss Barrel, whereas if you had Gauss Barrel at the start, you'd be losing out on those extra auto attacks. The reason why I included hard casting Gauss Barrel is because you're going to have to do that every time with your buffs active before Wildfire is used.
EDIT: Actually, using Gauss Barrel pre-pull and following it up with Gauss Round right at the start results in higher damage, thanks for the idea!
