  The linear and nonlinear evolutions of the tearing instability in a
collisionless plasma with a strong guide field are analyzed on the basis of a
two-field Hamiltonian gyrofluid model. The model is valid for a low ion
temperature and a finite $\beta_e$. The finite $\beta_e$ effect implies a
magnetic perturbation along the guide field direction and electron finite
Larmor radius effects. A Hamiltonian derivation of the model is presented. A
new dispersion relation of the tearing instability is derived for the case
$\beta_e=0$ and tested against numerical simulations. For $\beta_e \ll 1$ the
equilibrium electron temperature is seen to enhance the linear growth rate,
whereas we observe a stabilizing role when electron finite Larmor radius
effects become more relevant. In the nonlinear phase, a double
"faster-than-exponential" growth is observed, similarly to what occurs in the
presence of ion finite Larmor radius effects. Energy transfers are analyzed and
the conservation laws associated with the Casimir invariants of the model are
also discussed. Numerical simulations seem to indicate that finite $\beta_e$
effects do not produce qualtitative modifications in the structures of the
Lagrangian invariants associated with Casimirs of the model.
