Similar to the slang word "cum" in that it functions as both a verb and a noun by describing both ejaculation and semen, respectively.
As a noun, it is inspired by the archaic Nickelodeon product, Gak. Thus its use is preferable if the load of semen is of a thicker consistency. On the other hand, if the load is of a watery, drippy consistency, then "squirt" is probably a better word.
As a verb, "gak" is obscure, which makes it handy for formal situations in which "bust a nut, cum, cream my jeans, blow my load," etc. would be considered socially unacceptable.
Rhymes with "clack," which is a slang word for carnal, aggressive, "balls-slapping-ass" sex. Thus, to "clack 'n gak" would be to suggest a "down and dirty" coital session in which the male only concerns himself with his own pleasure and ultimately, his ejaculation.
"I gakked in her hair once, and since I felt so badly I bought her some Gak."
"Oh God...I'm about to gak!"
"There's still gak all over your earlobe."
"A quick clack n' gak, and then I was out of there."