Guinsoo's kog is extremely overrated and is not the most efficient build for Kog'maw
is bad because it's too hard to benefit from the max stacks, and it is a purely offensive item (does not offer lifesteal, mobility, etcetera).
In my opinion, the most efficient build is one which simply skips guinsoo's
This build is:
(sell
and buy
after guardian angel)

Runes are also important, I have found the best runes with this build to be: Primary Lethal tempo + triumph + legend: bloodline + coup de grace Secondary taste of blood + ravenous hunter
Runes are designed to get you out of early game with a lot of innate sustain, and lethal tempo helps you better scale hard into late game to compensate trading off damage for more defensive items.