We probe the structure and electronic band structure of graphene grown on a Pt(100) substrate using scanning tunneling microscopy, low energy electron diffraction, and angle-resolved photoemission spectroscopy. It is found that the graphene layer lacks a well-defined azimuthal orientation with respect to the substrate, causing a circular smearing of the π band instead of a well-defined Dirac cone near the Fermi level. The graphene is found to be electron doped placing the Dirac point ∼0.45 eV below the Fermi level, and a gap of 0.15±0.03 eV is found at the Dirac point. We dose atomic hydrogen and monitor the coverage on the graphene by analyzing the impurity-induced broadening of the π-band width. Saturation of graphene on Pt(100) with hydrogen does not expand the band gap, but instead hydrogen-mediated broadening and rehybridization of the graphene sp2 bonds into sp3 leads to a complete disruption of the graphene π band, induces a lifting of the Pt(100) reconstruction, and introduces a dispersing Pt state near the Fermi level. Deposition of rubidium on graphene on Pt(100) leads to further electron doping, pushing the Dirac point to a binding energy of ∼1.35 eV, and increasing the band gap to 0.65±0.04 eV.