Sorry, I don't use guides to play League. Ruins the fun of finding a good rune/mastery/playstyle.
BoTK is a counter to warmogs.
4% of their current health in bonus AD? You can see why it's a counter to warmogs.
GA is purely made for having that extra life for being targeted too much.
As an ADC I have never needed to go for a GA that's still in team-fights. Only when I do final push is when I decide to pull out the GA.