  We report the structural and magnetic characterization of La-substituted
Gd$_{1-x}$La$_x$RhIn$_5$ ($x\leq$ 0.50) antiferromagnetic (AFM) compounds. The
magnetic responses of pure GdRhIn$_5$ are well described by a $S=7/2$
Heisenberg model. When Gd$^{3+}$ ions are substituted by La$^{3+}$, the maximum
of the susceptibility and the inflection point of the magnetic specific heat
are systematically shifted to lower temperatures accompanied by a broadening of
the transition. The data is qualitatively explained by a phenomenological model
which incorporates a distribution of magnetic regions with different transition
temperatures ($T_N$). The universal behaviour of the low temperature specific
heat is found for La (vacancies) concentrations below $x=0.40$ which is
consistent with spin wave excitations. For $x=0.5$ this universal behaviour is
lost. The sharp second order transition of GdRhIn$_5$ is destroyed, as seen in
the specific heat data, contrary to what is expected for a Heisenberg model.
The results are discussed in the context of the magnetic behavior observed for
the La-substituted (Ce,Tb,Nd)RhIn$_5$ compounds.
