This is an announcement to the community to test these things out that might not know about them.
Hextech Gunblade no longer has Life Steal or Spell Vamp and has a new passive that heals you for 15% of ALL damage dealt, meaning this is Life Steal and Spell Vamp BUT it also heals for on-hit magic damage allowing AP Marksmen to be a thing have a dedicated "life steal" item for themselves, this is something they desperately needed to become viable is sustain health sustain from their damage type; since they are basic attacking they fully use the Attack Damage on the item.
Guinsoo's Rageblade also got changed, still granting hybrid stats; the passive now also grants bonus attack damage per stack along with ability power and attack speed [these two are unchanged]. At maximum stacks you deal bonus magic damage on-hit. So a great item for AP marksmen being basic attackers allows them to use attack damage well.
AP marksmen are those like AP Teemo and AP Varus, and can be in the duo lane with the support. Kog'Maw of course can use the same build and play style, for Kog'Maw the AP marksman play style is a lot different than the mage Kog'Maw so this is something to be aware of.
The build is something like:
and if you want: Runaan's Hurricane (be nice if it was more attack speed focused, keeping the 2500g cost) or just a defensive item or anything you feel like here; can go Deathcap for more damage
Boots: either Grieves or Sorcerers - depending if you want magic pen or attack speed