A graft copolymer comprising a backbone polymer and a branched-chain polymer, and represented by formula (I), where A, B, Ra, Rb, Rc, Rd, Re, Rf, G1, G2, G3, G4, Y1, Y2, and k are as defined in the specification. A process for producing the grate copolymer, a process for preparing a gel polymer electrolyte including the graft copolymer, and an intermediate copolymer of the graft copolymer are also disclosed.