  We have measured the donor-bound electron spin dynamics in cubic GaN by
time-resolved Kerr rotation experiments. The ensemble electron spin dephasing
time in this quantum dot like system characterized by a Bohr radius of 2.5 nm
is of the order of 1.5 ns as a result of the interaction with the fluctuating
nuclear spins. It increases drastically when an external magnetic field as
small as 10 mT is applied. We extract a dispersion of the nuclear hyperfine
field {\delta}Bn $\sim$ 4 mT, in agreement with calculations. We also
demonstrate for the first time in GaN based systems the optical pumping of
nuclear spin yielding the build-up of a significant nuclear polarization.
