Thermodynamic and electrical transport properties are reported for single crystals of the trigonal ${\mathrm{GdAl}}_2{\mathrm{Ge}}_2$, where complex antiferromagnetic ordering is seen at $T_{N1}$ = 23.3 K and $T_{N2}$ = 20.8 K and a rich phase diagram is revealed under applied magnetic fields. Angle-resolved photoemission spectroscopy (ARPES) measurements provide insights into this behavior by revealing a threefold symmetric Fermi surface that reflects the trigonal space-group symmetry. This promotes competing long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between localized magnetic moments and is associated with the appearance of fine tuned ordered states. This is accompanied by intrinsic null orbital angular momentum ($L$ $=$ 0 for trivalent Gd), which yields Heisenberg spins and ensures that the symmetry of the Fermi surface dominates the magnetic behavior over wide temperature ranges. Comparisons are made to related triangular-net $f$-electron materials, providing guidance for understanding their diverse behaviors.

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• Magnetic interactions
• Magnetic order
• Magnetic phase transitions
• Magnetic texture
• Magnetotransport
• Phase diagrams