  In intermetallic compounds with zero-orbital momentum ($L=0$) the magnetic
anisotropy and the electronic band structure are interconnected. Here, we
investigate this connection on divalent Eu and trivalent Gd intermetallic
compounds. We find by X-ray magnetic circular dichroism an out-of-plane easy
magetization axis in 2D atom-thick EuAu$_2$. Angle-resolved photoemission and
density-functional theory prove that this is due to strong $f-d$ band
hybridization and Eu$^{2+}$ valence. In contrast, the easy in-plane
magnetization of the structurally-equivalent GdAu$_2$ is ruled by
spin-orbit-split $d$-bands, notably Weyl nodal lines, occupied in the Gd$^{3+}$
state. Regardless of the $L$ value, we predict a similar itinerant electron
contribution to the anisotropy of analogous compounds.
