  The nonlinear dynamics of collisionless reconnecting modes is investigated,
in the framework of a three-dimensional gyrofluid model. This is the relevant
regime of high-temperature plasmas, where reconnection is made possible by
electron inertia and has higher growth rates than resistive reconnection. The
presence of a strong guide field is assumed, in a background slab model, with
Dirichlet boundary conditions in the direction of nonuniformity. Values of ion
sound gyro-radius and electron collisionless skin depth much smaller than the
current layer width are considered. Strong acceleration of growth is found at
the onset to nonlinearity, while at all times the energy functional is well
conserved. Nonlinear growth rates more than one order of magnitude higher than
linear growth rates are observed when entering into the small-$\Delta'$ regime.
