A sexual act which involves removing your penis from a girls vagina, ejaculating in your hand and feeding it to the girl. The sound that is made while the girl is eating said sperm from your hand, should be similar to the sound of a gerbal eating its food.... hence the name. There is different variations of gerbals such as, the "waterfall", "spicy gerbal", the "typewritter", etc. After the act speaking or seeing the girl afterwards is frowned apon due to the level of disgust of the girl and lack of respect.
last night i gerbaled this girl i met from the bar. i came in my hand and she totally ate every drop of cum off of it!