Mineral–polymer composite materials have been used as artificial bone grafts and scaffolds in bone tissue engineering. Polymer-controlled mineralization is effective for fabricating such composites. In this study, we synthesized organic–inorganic composites using anionic gelatin methacrylate (GelMA) hydrogels containing a high percentage of Ca²⁺ binding-carboxyl groups as a template for mineralization. A homogeneous surface and interior carbonated hydroxyapatite were achieved on the resulting mineralized porous hydrogel composites, and they were confirmed to resemble apatite-like structures. The effect of crosslinker content on mineralization was examined using GelMA hydrogels with different degrees of methylacrylation (DM). It was found that increasing the DM of the hydrogel suppressed the growth of carbonated hydroxyapatite layers, as was evident from the extent of calcification and the morphology of the minerals. The dependency of the mineralization on hydrogel variables was related to the changes in physicochemical properties of gel, including charge density and swelling. Compressive mechanical testing demonstrated that the compressive modulus and strength of the hydrogels increased with increasing DM and mineralization extent. Overall, mineralization of GelMA hydrogels with controllable mineral content and good mechanical properties provides a biomimetic route toward the development of bone substitutes for the next generation of biomaterials. The results of this study also provide insight into better understanding the role of the hydrogel matrix in biomineralization.