Similar to bukkake (which involves often a chick or sometimes two being ejaculated on repeatedly in succession by a series of guys - usually on the face or ass), gukkon involves multiple ejaculation directly into the mouth and generally followed by a large gulping or swallowing. "Gukkon" is an onomatopoeia, translated into English as "gulp" and is the sound made when downing a mouthful of cum
"Hey man did you check out that new bukkake movie?"
"nah, i'm into the gukkon shit. Its so hot when they swallow 20 guys' loads at once"