This word is often used as slang among netizens who are Chinese youngster or youth on the website. It is composed of two language; Chinese and English, which is called "Chinglish, Chinese English."

geli (给力) means "give power to, awesome etc."

-able is a suffix that turns words into a form of adjective. word "believable" is the model of the "-able" term.

so term "gelivable" means "awesome, fantastic, incredible, cool, etc."