This study explores the coexistence of magnetism, ferroelectricity, and ferrovalley polarization in two-dimensional (2D) materials, focusing on bilayer GdI₂. The research demonstrates that interlayer sliding can manipulate these properties, enabling tunable and reversible magnetic and valley polarization. Monolayer GdI₂ exhibits a ferromagnetic semiconductor with a valley polarization of up to 155.5 meV. Bilayer GdI₂ shows coexisting magnetism, ferroelectricity, and ferrovalley polarization, which can be controlled by interlayer sliding. The magnetic phase transition is explained through spin Hamiltonian and electron hopping between layers. The study reveals that the stacking order significantly influences the magnetic ground state, ferroelectric polarization, and valley polarization. The AA stacking of GdI₂ has an antiferromagnetic (AFM) ground state, while sliding to AB or BA stacking induces a ferromagnetic (FM) state with spontaneous valley and ferroelectric polarization. The AB and BA stacking can be interconverted by sliding, switching the ferroelectric polarization between +z and -z directions. The study also shows that the valley polarization can be effectively tuned by switching the magnetization direction. The magnetic ground state of GdI₂ is influenced by the stacking order, with AA stacking favoring AFM and AB/BA stacking favoring FM. The interlayer exchange interactions depend on the stacking order, with AFM coupling in AA stacking and FM coupling in AB/BA stacking. The study provides a new design for 2D multiferroics, offering a practical way to design advanced valleytronic and spintronic devices through the coupling of multiple ferroic orders. The findings highlight the potential of 2D multiferroics in next-generation electronic and spintronic applications.This study explores the coexistence of magnetism, ferroelectricity, and ferrovalley polarization in two-dimensional (2D) materials, focusing on bilayer GdI₂. The research demonstrates that interlayer sliding can manipulate these properties, enabling tunable and reversible magnetic and valley polarization. Monolayer GdI₂ exhibits a ferromagnetic semiconductor with a valley polarization of up to 155.5 meV. Bilayer GdI₂ shows coexisting magnetism, ferroelectricity, and ferrovalley polarization, which can be controlled by interlayer sliding. The magnetic phase transition is explained through spin Hamiltonian and electron hopping between layers. The study reveals that the stacking order significantly influences the magnetic ground state, ferroelectric polarization, and valley polarization. The AA stacking of GdI₂ has an antiferromagnetic (AFM) ground state, while sliding to AB or BA stacking induces a ferromagnetic (FM) state with spontaneous valley and ferroelectric polarization. The AB and BA stacking can be interconverted by sliding, switching the ferroelectric polarization between +z and -z directions. The study also shows that the valley polarization can be effectively tuned by switching the magnetization direction. The magnetic ground state of GdI₂ is influenced by the stacking order, with AA stacking favoring AFM and AB/BA stacking favoring FM. The interlayer exchange interactions depend on the stacking order, with AFM coupling in AA stacking and FM coupling in AB/BA stacking. The study provides a new design for 2D multiferroics, offering a practical way to design advanced valleytronic and spintronic devices through the coupling of multiple ferroic orders. The findings highlight the potential of 2D multiferroics in next-generation electronic and spintronic applications.