The present invention is directed to a gel polymer electrolyte for use in rechargeable polymer secondary batteries and a precursor composition thereof. The precursor composition can be injected into an aluminum shell of a battery cell, which undergoes in-situ heating polymerization by heating and forms a gel polymer electrolyte penetrating a partition membrane therein. The precursor composition contains (meth)acrylic (acrylate) monomers and a modified bismaleimide oligomer resulting from a reaction of barbituric acid and bismaleimide.