Based on first-principles calculations, we systematically investigate the electronic and optical properties of van der Waals (vdW) heterostructures composed of graphene-like gallium nitride (g-GaN) and transition metal dichalcogenides (TMDs). The investigated vdW heterostructures (g-GaN/MoS2, g-GaN/WS2, g-GaN/MoSe2, and g-GaN/WSe2) are all semiconductors with direct bandgap. In particular, both the g-GaN/MoS2 and g-GaN/WS2 vdW heterostructures possess type-II band alignment, which will facilitate the separation of photogenerated carriers, and enhance their lifetime. Furthermore, band edge positions of these two heterostructures satisfied both water oxidation and reduction energy requirements, suggesting the potential in photocatalysts for water splitting. In addition, both g-GaN/MoS2 and g-GaN/WS2 vdW heterostructures exhibit a high electron mobility, which ensure that the redox reactions for water splitting will be effectively proceeded. More importantly, they show significant absorption peaks in the visible light region, leading to highly efficient utilization of the solar energy. These fascinating properties render the g-GaN/MoS2 and g-GaN/WS2 vdW heterostructures high-efficiency photocatalysts for water splitting.