  We explain a profound complexity of magnetic interactions of some
technologically relevant gadolinium intermetallics using an ab-initio
electronic structure theory which includes disordered local moments and strong
$f$-electron correlations. The theory correctly finds GdZn and GdCd to be
simple ferromagnets and predicts a remarkably large increase of Curie
temperature with pressure of +1.5 K kbar$^{-1}$ for GdCd confirmed by our
experimental measurements of +1.6 K kbar$^{-1}$. Moreover we find the origin of
a ferromagnetic-antiferromagnetic competition in GdMg manifested by
non-collinear, canted magnetic order at low temperatures. Replacing 35\% of the
Mg atoms with Zn removes this transition in excellent agreement with
longstanding experimental data.
