We report structural, surface, and magnetic investigations of ferromagnetic Ga-rich MnGa thin and ultra thin films grown on semiconducting GaN(0001) using molecular beam epitaxy. The Mn:Ga composition ratio is varied from 1 (stoichiometric) to approximate to 0.42 (very Ga-rich) for different samples. We find that the L1(0) MnGa phase is preserved down to a Mn: Ga ratio of approximate to 0.81. As the Ga concentration increases, we observe the coexistence of more Ga-rich phases, namely Mn3Ga5 and Mn2Ga5. Room temperature scanning tunneling microscopy imaging reveals highly epitaxial films, with atomically smooth and highly reconstructed surfaces. Magnetic characterizations show how the magnetic properties evolve with changing composition and that giant perpendicular magnetic anisotropy is induced by reducing the size of our films. (C) 2016 Elsevier B.V. All rights reserved.