Altermagnetism is a recently identified magnetic phase characterized by symmetry-enforced nonrelativistic spin splitting despite vanishing net magnetization. Here, using spin-space symmetry analysis and first-principles calculations, we show that inversion-symmetry-broken stackings of bilayer ${\mathrm{GdI}}_3$, a two-dimensional van der Waals antiferromagnet with localized Gd $4f$ moments, host stacking-tunable altermagnetism. While the magnetic order originates from localized $4f$ electrons, the altermagnetic spin splitting is carried by I $p$ states near the Fermi level, revealing a separation between the source of magnetism and the electronic states that encode the spin texture. Specific stackings, namely, ${\mathrm{AB}}^{*}/{\mathrm{BA}}^{*}$ and ${\mathrm{AC}}^{*}/{\mathrm{CA}}^{*}$, realize symmetry-protected altermagnetic phases, whereas the ${\mathrm{AA}}^{*}$ stacking remains spin degenerate. Moreover, the ${\mathrm{AB}}^{*}/{\mathrm{BA}}^{*}$ stackings simultaneously develop out-of-plane ferroelectric polarization, while the ${\mathrm{AC}}^{*}/{\mathrm{CA}}^{*}$ stackings remain nonferroelectric due to cancellation of local dipoles. Upon including spin-orbit coupling, the relevant antiunitary symmetries are lifted and the system transitions from an altermagnetic regime to a Rashba-type spin-split state with a rich momentum-space spin texture, while the total magnetization remains nearly zero. Our results establish bilayer ${\mathrm{GdI}}_3$ as a stacking-tunable platform for exploring the symmetry connection between altermagnetism, ferroelectricity, and relativistic spin textures.

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• Altermagnetism
• Ferroelectricity
• Altermagnets
• Antiferromagnets
• Monolayer films
• van der Waals systems
• Density functional calculations
• First-principles calculations