To protect the quantum anomalous Hall state from thermal fluctuation, it is necessary to search for magnetic topological insulators with giant magnetic anisotropy. We have investigated the magnetic and topological properties of Os-based dimers adsorbed onto graphene by density-functional calculations plus the Wannier-function technique. Our results reveal that the systems of Os-Fe@G, Os-Ru@G, and Os-Rh@G possess both giant magnetic anisotropy, typically larger than 100 meV, and a large nontrivial topological band gap. Both the magnetic anisotropy and the band gap can be effectively tuned by the electric field. The dimers prefer ferromagnetic coupling and are stable against thermal fluctuation. The topological band-gap opening can be attributed to the intrinsic spin-orbital coupling of the dimer rather than Rashba spin-orbital coupling. These Chern insulators are found to be [script C] = ±2 with two conducting edge states crossing the Fermi level.