Magnetic order engineering in two-dimensional Dirac systems is of great interest for theoretical and technological exploration. Up to now, the experimental advances in this field mostly concerned graphene monolayers. Here, we report a comprehensive study of a monolayer-thick germanene-like sheet in contact with gadolinium atoms. Direct observations supported by first-principles calculations reveal the fingerprints of the Dirac fermions in the electronic structure and noncollinear antiferromagnetism. The hybridization of the germanene layer with Gd atoms leads to a large and tunable gap in the Dirac states that carry a nonzero spin-Berry curvature. We discovered that cesium-induced controlled electron doping can switch the system into a ferromagnetic state and then back to the antiferromagnetism at saturated cesium monolayer limit. We explain these reversible magnetic transitions by the oscillatory behavior of the Ruderman-Kittel-Kasuya-Yosida interaction and suggest that this system could find application in magnetoelectronics and spintronics.