Graphene has been recognized as a promising 2D material with many new properties. However, pristine graphene is gapless which hinders its direct application towards graphene-based semiconducting devices. Recently, various ways have been proposed to overcome this problem. In this study, we report a robust method to open a gap in graphene via noncovalent functionalization with porphyrin molecules. Two type of porphyrins, namely, iron protoporphyrin (FePP) and zinc protoporphryin (ZnPP) were independently physisorbed on graphene grown on nickel by chemical vapour deposition (CVD) resulting in a bandgap opening in graphene. Using a statistical analysis of scanning tunneling spectroscopy (STS) measurements, we demonstrated that the magnitude of the band gap depends on the type of deposited porphyrin molecule.The π-π stacking of FePP on graphene yielded a considerably larger band gap value (0.45 eV) than physisorbed ZnPP (0.23 eV). We proposed that the origin of different band gap value is governed due to the metallic character of the respective porphyrin.