  Five-dimensional collective Hamiltonian based on the covariant density
functional theory has been applied to study the the low-lying states of
even-even $^{148-162}$Gd isotopes. The shape evolution from $^{148}$Gd to
$^{162}$Gd is presented. The experimental energy spectra and intraband $B(E2)$
transition probabilities for the $^{148-162}$Gd isotopes are reproduced by the
present calculations. The relative $B(E2)$ ratios in present calculations are
also compared with the available interacting boson model results and
experimental data. It is found that the occupations of neutron $1i_{13/2}$
orbital result in the well-deformed prolate shape, and are essential for Gd
isotopes.
