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.
"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."