  Enthalpies of formation of gallium interstitials and all the other native
point defects in gallium arsenide are calculated using the same well-converged
\emph{ab initio} techniques. Using these results, equilibrium concentrations of
these defects are computed as a function of chemical potential from the arsenic
rich limit to the gallium rich limit and as a function of the doping level from
$p$-type to $n$-type. Gallium interstitial diffusion paths and migration
barriers for diffusion are determined for all the interstitial charge states
which are favored for Fermi levels anywhere in the gap, and the charge states
which dominate diffusion as a function of Fermi level are identified. The
effects of chemical potential, doping level, and non-equilibrium defect
concentrations produced by ion implantation or irradiation on gallium
self-diffusion are examined. Results are consistent with experimental results
across the ranges of doping and stoichometry where comparisons can be made.
Finally, these calculations shed some light on the complex situation for
gallium diffusion in gallium arsenide that is gallium-rich and doped heavily
$p$-type.
