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