  In this work we report the appearence of a large perpendicular magnetic
anisotropy (PMA) in Fe$_{1-x}$Ga$_x$ thin films grown onto ZnSe/GaAs(100). This
arising anisotropy is related to the tetragonal metastable phase in as-grown
samples recently reported [M. Eddrief {\it et al.}, Phys. Rev. B {\bf 84},
161410 (2011)]. By means of ferromagnetic resonance studies we measured PMA
values up to $\sim$ 5$\times$10$^5$ J/m$^3$. PMA vanishes when the cubic
structure is recovered upon annealing at 300$^{\circ}$C. Despite the important
values of the magnetoelastic constants measured via the cantilever method, the
consequent magnetoelastic contribution to PMA is not enough to explain the
observed anisotropy values in the distorted state. {\it Ab initio} calculations
show that the chemical ordering plays a crucial role in the appearance of PMA.
Through a phenomenological model we are able to explain that an excess of next
nearest neighbour Ga pairs (B$_2$-like ordering) along the perpendicular
direction arises as the source of PMA in Fe$_{1-x}$Ga$_x$ thin films.
