You won't usually get to a full 6 item build because the armor pen trio is so expensive.
Therefore, save GA for last unless you are constantly being bursted by physical damage.
Maw and Mercurial will help with AP by providing plenty of magic resist and lifesteal, plus Maw's passive shield against Magic Damage.
Build Maw first for more damage or Mercurial if you are getting locked down by slows, stuns, roots, etc.
If the game goes past 40-50 min and there is no clearly stronger side, you need to figure out whether or Bloodthirster are more important to you.
If you can't stop getting caught because the opponent can easily jump on you in fights, then you will need GA.
is much more helpful in surviving long fights and goes well with a black cleaver.
is a great substitute for one of your armor pen items.
It's useful when your team has trouble killing the enemy tank(s) and you are not the only big AD source because it applies 30% armor reduction.
This means instead of you building or for your own 45% bonus armor penetration, you can give your entire team up to 30% bonus armor penetration on one target to more quickly kill someone like malphite or Ramus who heavily stacks armor and is much harder to kill late game.