  The transverse-momentum ($Q_T$) distribution of low-mass Drell-Yan pairs is
calculated in QCD perturbation theory with all-order resummation of $\alpha_s
(\alpha_s \ln(Q^2_T/Q^2))^n$ type terms. We demonstrate that the rapidity
distribution of low-mass Drell-Yan pairs at large-enough transverse momentum is
an advantageous source of constraints on the gluon distribution and its nuclear
dependence. We argue that low-mass Drell-Yan pairs in the forward region
provide a good and clean probe of small-$x$ gluons at collider energies.
