A highly symmetric structure of metal dimers embedded in defect-graphene (M₂⊥gra) in a perpendicular manner was designed. Five M₂⊥gra (M = Co, Ni, Rh, Ir and Pt) monolayers were identified to be stable by density functional theory (DFT) calculations. To investigate the capability of those new structures as gas sensors, the adsorption behavior of ten gas molecules (O₂, N₂, CO, CO₂, NO, NO₂, NH₃, H₂O, H₂S and SO₂) on M₂⊥gra was explored, and the charge transfer, magnetism changes, etc. of these adsorption systems were analyzed. The Ni₂⊥gra can be used as a gas sensor for O₂ at 500 K by the analysis of electronic and magnetic properties. At room temperature, the Pt₂⊥gra is expected to be an excellent CO₂ gas selector due to its high selectivity, sensitivity and short recovery time (1.04 × 10⁻¹² s). The electronic and magnetic coupling between the metal atoms in the vertical metal dimers plays an important role in sensing gas molecules. Our work paves a new way to design metal-dimer-based nanomaterials.