  Motivated by the recent experimental observation of long range ferromagnetic
order at a relatively high temperature of 200K in the Fe-doped ZnGa$_2$O$_4$
semiconducting spinel, we propose a possible mechanism for the observed
ferromagnetism in this system. We show, supported by band structure
calculations, how a model similar to the double exchange model can be written
down for this system and calculate the ground state phase diagram for the two
cases where Fe is doped either at the tetrahedral position or at the octahedral
position. We find that in both cases such a model can account for a stable
ferromagnetic phase in a wide range of parameter space. We also argue that in
the limit of high Fe$^{2+}$ concentration at the tetrahedral positions a
description in terms of a two band model is essential. The two $e_g$ orbitals
and the hopping between them play a crucial role in stabilizing the
ferromagnetic phase in this limit. The case when Fe is doped simultaneously at
both the tetrahedral and the octahedral position is also discussed.
