  Dimensionality can strongly influence the magnetic structure of solid
systems. Here, we predict theoretically and confirm experimentally that the
antiferromagnetic (AFM) ground state of bulk gadolinium germanide metalloxene,
which has a quasi-layered defective GdGe$_2$ structure, is preserved in the
ultrathin film limit. \textit{Ab initio} calculations demonstrate that
ultrathin GdGe$_2$ films present in-plane intra-layer ferromagnetic coupling
and AFM inter-layer coupling in the ground state. Angle-resolved photoemission
spectroscopy finds the AFM-induced band splitting expected for the 2 and 3
GdGe$_2$ trilayer (TL) films, which disappear above the N\'eel temperature. The
comparative analysis of isostructural ultrathin DyGe$_2$ and GdSi$_2$ films
confirms the magnetic origin of the observed band splitting. These findings are
in contrast with the recent report of ferromagnetism in ultrathin metalloxene
films, which we ascribe to the presence of uncompensated magnetic moments.
