Square-lattice moiré systems provide a promising route for quantum simulation of Hubbard physics, yet electrically tunable realizations of the hopping ratio t'/t have so far been limited to specific valley configurations. A natural question is whether the same tunability can be extended to the more broadly occurring Γ-valley systems. Here we identify a symmetry-controlled route for realizing electrically tunable square-lattice Hubbard models in Γ-valley twisted homobilayers. At small twist angles, an emergent layer-exchange symmetry separates the low-energy states into flat bands localized on two nested square sublattices, suppressing inter-sublattice hopping. An interlayer displacement field breaks this symmetry and induces controllable hybridization between the sublattices, enabling continuous tuning of the t'/t ratio over a wide range while preserving an effective single-band description. We further establish a formal correspondence between Γ- and M-valley moiré systems, revealing them as different symmetry limits of a unified framework for tunable square-lattice Hubbard models. Our results uncover a general symmetry principle underlying displacement-field tunability and extend electrically controllable square-lattice Hubbard physics to a broad family of Γ-valley materials.