  In this paper we consider the parametrizations of gluon transverse momentum
dependent (TMD) correlators in terms of TMD parton distribution functions
(PDFs). These functions, referred to as TMDs, are defined as the Fourier
transforms of hadronic matrix elements of nonlocal combinations of gluon
fields. The nonlocality is bridged by gauge links, which have characteristic
paths (future or past pointing), giving rise to a process dependence that
breaks universality. For gluons, the specific correlator with one future and
one past pointing gauge link is, in the limit of small $x$, related to a
correlator of a single Wilson loop. We present the parametrization of Wilson
loop correlators in terms of Wilson loop TMDs and discuss the relation between
these functions and the small-$x$ `dipole' gluon TMDs. This analysis shows
which gluon TMDs are leading or suppressed in the small-$x$ limit. We discuss
hadronic targets that are unpolarized, vector polarized (relevant for
spin-$1/2$ and spin-$1$ hadrons), and tensor polarized (relevant for spin-$1$
hadrons). The latter are of interest for studies with a future Electron-Ion
Collider with polarized deuterons.
