  We report on the fluidization dynamics of an attractive gel composed of
non-Brownian particles made of fused silica colloids. Extensive rheology
coupled to ultrasonic velocimetry allows us to characterize the global stress
response together with the local dynamics of the gel during shear startup
experiments. In practice, after being rejuvenated by a preshear, the gel is
left to age during a time $t_w$ before being submitted to a constant shear rate
$\dot \gamma$. We investigate in detail the effects of both $t_w$ and $\dot
\gamma$ on the fluidization dynamics and build a detailed state diagram of the
gel response to shear startup flows. The gel may either display transient shear
banding towards complete fluidization, or steady-state shear banding. In the
former case, we unravel that the progressive fluidization occurs by successive
steps that appear as peaks on the global stress relaxation signal. Flow imaging
reveals that the shear band grows up to complete fluidization of the material
by sudden avalanche-like events which are distributed heterogeneously along the
vorticity direction and correlated to large peaks in the slip velocity at the
moving wall. These features are robust over a wide range of values of $t_w$ and
$\dot \gamma$, although the very details of the fluidization scenario vary with
$\dot \gamma$. Finally, the critical shear rate $\dot \gamma^*$ that separates
steady-state shear-banding from steady-state homogeneous flow depends on the
width on the shear cell and exhibits a nonlinear dependence with $t_w$. Our
work brings about valuable experimental data on transient flows of attractive
dispersions, highlighting the subtle interplay between shear, wall slip and
aging which modeling constitutes a major challenge that has not been met yet.
