Great Ape Cell is a transformation of a Great Ape, that Cell is able to take due to the fact he is made up with all Saiyan cells. This transformation takes place once his tail is cut off and his Saiyan tail appears. This Saiyan tail generates Cybro Waves that once Cell sets eyes on a Saiyan, he transforms into an Oozaru (Great Ape). Cell has the ability to control his Great Ape fully and can only be turned back to normal by cutting off his tail, thus bringing back Cell's old tail.
The appearance of a Great Ape Cell is quite similar to a Great Ape but its fur color and pattern takes more of Cell's likeness.
This tranformation increases Cell's power level by four times: 520,000,000.