Graph manifolds are manifolds that decompose along tori into pieces with a tame S 1 -structure. In this paper, we prove that the simplicial volume of graph manifolds (which is known to be zero) can be approximated by integral simplicial volumes of their finite coverings. This gives a uniform proof of the vanishing of rank gradients, Betti number gradients and torsion homology gradients for graph manifolds. Theorem 1.2. Let M be an oriented closed connected graph manifold (in the sense of Definition 2.7) with residually finite fundamental group. Then M = M ∞ Z = 0. More generally, let (Γ j ) j∈N be a descending chain of finite index subgroups of π 1 (M ) with trivial intersection and let (M j ) j∈N be the corresponding tower of finite coverings. Then M = lim j→∞ M j Z [π 1 (M ) : Γ j ] = 0.