Gak is a Nickelodeon putty toy, a compound of variable colors, that has been sold and distributed from 1992 to the present day.
Origin
The Gak meme initially originated on livestreams of the first two episodes of the third season of My Little Pony: Friendship is Magic, where the commercial was seen several times repetitively during the airing. Many Bronies found this amusing/annoying and began to flood the livestream chat with Gak-related entries.
The following is recorded footage of said livestream and its chat from BronyState (note how in 33:50, when the commercial begins, the chat is almost instantly flooded with “Gak”):
“Gak” subsequently appeared on 4chan, with threads like this, particularly related to the MLP livestream and the Gak commercials.
One of the earliest images (posted on Otaku Ascended during livestream) of Gak:
Spread
At 11:16 AM of November 10, the Gak was featured on the blog Equestria Daily by its creator Sethisto, who acknowledged the meme in this post.
Subsequently, Ask Gak on Tumblr was created.