The 24-year-old is currently with Standard Liege but has made only one appearance after being injured in pre-season.
Yesterday, Gershon's agent Dudu Dahan said: "We close everything this morning and now Rami will travel to Paris to sign his passport to the work licence."
Gershon added: "I'm very happy to sign for this fantastic club and I'm ready for the great challenge."
Neil Lennon is keen to add to his defensive options with Gershon able to play in central defence or at left-back.
Celtic could be without Efe Ambrose for up to a month with the defender currently on international duty at the African Nations Cup for Nigeria.
With Kelvin Wilson about to sit out a two-match domestic suspension, Lennon will be without his first-choice defensive partnership when Celtic return from their winter break to face Hearts on January 19.