Two-dimensional magnets provide fertile platforms for achieving high-temperature ferromagnetism, desirable perpendicular magnetic anisotropy, and tunable topological transport, enabling robust magnetic stability and efficient spin-charge conversion in spintronic devices. However, these properties are usually weakened by substrates. Here, we systematically study supported $\mathrm{Gd}{M}_2/M$(111) ($M=\mathrm{Cu}$, Ag, Au) surface alloys and freestanding monolayer $\mathrm{Gd}{M}_2$, using first-principles calculations. For supported systems, substrate-induced structural buckling and charge redistribution weaken their $M$-Gd (s, p)-d-mediated exchange interactions, leading to weakened ferromagnetism and low Curie temperatures (below 100 K). Upon removing the substrates, charge redistribution enhances the effective exchange couplings in freestanding monolayer $\mathrm{Gd}{M}_2$, and the elimination of buckling further strengthens ferromagnetism, thereby yielding Curie temperatures above room temperature. Notably, freestanding monolayer $\mathrm{Gd}{\mathrm{Au}}_2$ exhibits a pronounced out-of-plane magnetic anisotropy driven by the orbital redistribution near the Γ point. Combined with symmetry-tunable band topology, this anisotropy gives rise to a sizable anomalous Hall conductivity in freestanding monolayer $\mathrm{Gd}{\mathrm{Au}}_2$. Our results establish the connection between interfacial coupling, lattice geometry, electronic structure, and magnetic and topological properties, enabling the engineering of two-dimensional high-temperature spintronic functionalities.

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• Anomalous Hall effect
• Electrical properties
• Exchange interaction
• Magnetic anisotropy
• Magnetism
• Room-temperature magnet calculations
• 2-dimensional systems
• Ferromagnets
• First-principles calculations