GoK's is an artifact short for Gem of the Kursed. Its a high end artifact that is farmed from killing kurse.
A good comparison would be an SOJ back in d2.
So an example is if something is worth 50 Gok's, most people woudn't trade 50 Goks directly since it would be an insane amount to store.
They would trade something of equivalent value. I believe a Lizard Formula is worth about 8-10 Goks each, so for an item worth 50 Gok's people might offer 5 Lizards plus or minus a couple goks or other artifacts.
A good place to familiarize yourself with is this thread on the official forum
https://forums.marvelheroes.com/discussion/227623/december-14-2015-guide-to-commonly-traded-items .
Of course these prices may not always reflect current market trends or how people value the items (High rolled/perfect items are usually worth an insane amount more then market prices i.e a near perfect GOK could be worth 100+ GoKs of equivalent value), but it's definitely a good base line and a good starting point on what you should keep or discard.
This post was edited by Zifnab13 on Dec 17 2015 12:46am