In this study, we examine the structural, optoelectronic, and photocatalytic properties of GaN-based van der Waals heterostructures (vdWHs) that incorporate halogens (Cl, Br) and chalcogens (Se, Te). Using first-principles calculations based on density functional theory, we analyze six different stacking configurations of these heterostructures. Our results show that the GaN-VHC vdWHs (where H = Cl, Br and C = Se, Te) are both dynamically and energetically stable. For solar cell applications, the GaN-VHSe heterostructures exhibit a direct type-I band alignment, while the GaN-VHTe structures show an indirect type-I band alignment. All GaN-VHC heterostructures display strong optical peaks across the visible, infrared, and ultraviolet regions, highlighting their potential for optoelectronic applications. We investigated the photocatalytic potential of these heterostructures and found that GaN-VClSe performs water splitting at pH = 0. While model I and model II can facilitate water splitting, and mainly support reduction at pH = 0 and oxidation at pH = 7. However, their type-I band alignment inherently limits overall photocatalytic activity.