GGS-groups: order of congruence quotients and Hausdorff dimension

If G is a GGS-group defined over a p-adic tree, where p is an odd prime, we calculate the order of the congruence quotients $G_n=G\mathrm{/}{\text{Stab}}_G(n)$ for every n. If G is defined by the vector $e=(e_1,\mathrm{.}\mathrm{.}\mathrm{.},e_{p-1})\in {\text{F}}_p^{p-1}$, the determination of the order of $G_n$ is split into three cases, according as e is non-symmetric, non-constant symmetric, or constant. The formulas that we obtain only depend on p, n, and the rank of the circulant matrix whose first row is e. As a consequence of these formulas, we also obtain the Hausdorff dimension of the closures of all GGS-groups over the p-adic tree.