  The gluon content of the proton is embodied by the unintegrated gluon
distribution (UGD), which has universal validity. In literature many models of
UGD have been proposed so far. The polarized $\rho$-meson leptoproduction at
HERA offers a nowadays unexplored testfield to discriminate among existing
models of UGD, via the comparison with theoretical predictions formulated in
the $\kappa$-factorization approach.
