The magnetic coupling in the GdT 2 Si 2 (T = transition metal) antiferromagnetic compounds is studied by means of DC susceptibility and high-field free-powder (HFFP) measurements. In most compounds, the experimental results can be described in the framework of a two-sublattice model without anisotropy. From this analysis, an intrasublattice exchange coupling constant, n 1 1 , and an intersublattice exchange coupling constant, n 1 2 , are obtained. The exchange couplings are discussed in terms of the change of the lattice parameters, the volume V and the filling of the transition metal electron band. The apparently reduced value of the moment per Gd atom in the forced ferromagnetic configuration is attributed to a reduction of the on-site 5d-moment and a possible negative polarisation of the other non-4f electrons.