To compute the multiplicative inverse modulo a prime one can use extended GCD or Euler's theorem:
$$x^{-1} = x^{p-2} \mod p$$
Is there similar formula for x being a Gaussian integer? I'm asking because
$$-i^{3-2} \mod 3 = -i$$
but Mathematica says:
PowerMod[-i, -1, 3] = i