Ion diamagnetic effects on collisionless magnetic reconnection are investigated by means of numerical simulations of a Hamiltonian gyrofluid model. The work is focused in particular on the effects of inhomogeneous density equilibria in the large $\Delta'$ regime. The linear growth rates predicted by asymptotic theory are recovered. Nonlinearly the island shape is strongly modified and the flow changes its typical four-cell structure into a simpler two cell one. Contrary to the resistive reconnection process, we find that in Hamiltonian (i.e. collisionless) reconnection the width of the final saturated island is independent from the diamagnetic effects and the magnetic island grows up to the equilibrium scale length.