It actually originated on livestreams of MLP S03E01 (Mainly the Otaku Ascended). During the commercial breaks there was always at least one commercial for Gak and in one case it repeated itself right when it first ended. Many of bronies found it amusing/annoying/just plain weird and flooded livestream chat with entries like ‘GAK! It’s Gaaaaaaak!‘, ’Gak is the new Smooze’, ‘Gak is best pony’, ‘The final villain of this season will be the Gak!’ or just with spamming of ‘GAAAK’.
One of the earliest images posted of Gak is http://www.memecreator.org/static/images/memes/731866.jpg