The electron-attachment-induced proton transfer in the guanine-cytosine (G:C) base pair is thought to be relevant to the issues of charge transport and radiation damage in DNA. However, our understanding on the reaction mainly comes from the data of isolated bases and base pairs, and the behavior of the reaction in the DNA duplex is not clear. In the present study, the proton-transfer reaction in reduced G:C stacks is investigated by quantum mechanical calculations with the aim to clarify how each environmental factor affects the proton transfer in G:C center dot-. The calculations show that while the proton transfer in isolated G:C center dot- is exothermic with a small energetic barrier, it becomes endothermic with a considerably enhanced energetic barrier in G:C stacks. The substantial effect of G:C stacking is proved to originate from the electrostatic interactions between the dipole moments of outer G:C base pairs and the middle G:C center dot-base-pair radical anion; the extent of charge delocalization is very small and plays little role in affecting the proton transfer in G:C center dot-. On the basis of the electrostatic model, the sequence dependence of the proton transfer in the ionized G:C base pair is predicted. In addition, the water molecules in the first hydration shell around G:C center dot- display a pronounced effect that facilitates the proton-transfer reaction; further consideration of bulk hydration only slightly lowers the energetic barrier and reaction energy. We also notice that the water arrangement around an embedded G:C center dot- is different from that around an isolated G:C center dot-, which could result in a very different solvent effect on the energetics of the proton transfer. In contrast to the important influences of base stacking and hydration, the effects of sugar-phosphate backbone and counterions are found to be minor. Our calculations also reveal that a G:C base pair embedded in DNA is capable of accommodating two excess electrons only in bulk hydration; the resultant G(N1-H)(-):C(N3+H)(-) dianion is stable and exists long enough to lead to DNA damage. The combination of the present results with the previous findings in literature suggests that the behaviors of charge transport and low-energy electron-induced damage in DNA are highly susceptible to the hydration level.