  Computer simulations were done of the mean square displacement (MSD) of
tracer particles in colloidal gels formed by diffusion or reaction limited
aggregation of hard spheres. The diffusion coefficient was found to be
determined by the volume fraction accessible to the spherical tracers
($\phi_a$) independent of the gel structure or the tracer size. In all cases,
critical slowing down was observed at $\phi_a\approx 0.03$ and was
characterized by the same scaling laws reported earlier for tracer diffusion in
a Lorentz gas. Strong heterogeneity of the MSD was observed at small $\phi_a$
and was related to the size distribution of pores.
