This study explores the coexistence of magnetism, ferroelectricity, and ferrovalley in two-dimensional (2D) materials, specifically focusing on bilayer GdI$_2$. The monolayer GdI$_2$ exhibits ferromagnetic semiconductor properties with a valley polarization of up to 155.5 meV. In the bilayer, the stacking order of the layers significantly influences the magnetic, ferroelectric, and valley polarization properties. By sliding the layers, the magnetic ground state can transition from antiferromagnetic (AFM) to ferromagnetic (FM), leading to spontaneous valley polarization and ferroelectric polarization. The study reveals that the magnetic phase transition is driven by spin Hamiltonian and electron hopping between layers. The findings offer a new direction for designing advanced valleytronic and spintronic devices, as the 2D multiferroic properties can be tuned by interlayer sliding, providing a flexible and controllable coupling of multiple ferroic orders.This study explores the coexistence of magnetism, ferroelectricity, and ferrovalley in two-dimensional (2D) materials, specifically focusing on bilayer GdI$_2$. The monolayer GdI$_2$ exhibits ferromagnetic semiconductor properties with a valley polarization of up to 155.5 meV. In the bilayer, the stacking order of the layers significantly influences the magnetic, ferroelectric, and valley polarization properties. By sliding the layers, the magnetic ground state can transition from antiferromagnetic (AFM) to ferromagnetic (FM), leading to spontaneous valley polarization and ferroelectric polarization. The study reveals that the magnetic phase transition is driven by spin Hamiltonian and electron hopping between layers. The findings offer a new direction for designing advanced valleytronic and spintronic devices, as the 2D multiferroic properties can be tuned by interlayer sliding, providing a flexible and controllable coupling of multiple ferroic orders.