The effect of Ca doping to melt-processed Gd-based bulk superconductors fabricated by the oxygencontrolled melt growth (OCMG) method was studied. First, we simply added CaCO 3 to the starting materials to dope Ca and observed an increase in the trapped field and the critical current density (J c ) up to, at least, 0.31 wt.% CaCO 3 . However, an additional oxygen annealing resulted in a decrease of J c in magnetic fields and in a monotonous exponential-like field dependence of J c . This suggests that oxygen deficiency had caused the enhancement of J c , although the same annealing process as the non-doped sample was applied. We also prepared samples by adding not only CaCO 3 but also BaCO 3 and CuO in a molar ratio of Ca:Ba:Cu = 1:2:3, and observed a large peak effect of J c up to at least 0.31 wt.% CaCO 3 . It is likely that this large peak effect is also caused by oxygen defects, although the superconducting transition temperature was significantly large. These results indicate that J c at 77 K can be significantly enhanced by introducing a proper amount of oxygen deficiency and compensating at the same time the decrease of carrier density by Ca doping.