Two-dimensional transition-metal dichalcogenides offer strong light-matter interaction but suffer from inefficient carrier separation and contact-related losses in photodetectors. Here, we demonstrate a high-gain WS₂/Fe₃GaTe₂ van der Waals heterostructure photodetector, where metallic Fe₃GaTe₂ serves as an active interfacial contact. The work-function mismatch, together with interfacial charge redistribution and asymmetric contact geometry, contributes to a built-in field that supports self-powered photodetection at zero bias. Under 450 nm illumination, the device delivers a zero-bias responsivity of 23.5 A/W and an apparent external quantum efficiency of 6.4 x 10³%. At -1 V biasing, the heterostructure exhibits photoresponse at 450, 520 and 633 nm, achieving a responsivity of 9.7 x 10³ A/W and a noise derived specific detectivity of 2.3 x 10¹3 Jones at 100 Hz under 450 nm illumination. The high photoresponse is attributed to interfacial carrier separation, efficient extraction, and a likely contribution from trap-assisted photogating in multilayer WS₂. These results establish Fe₃GaTe₂-enabled interface engineering as an effective route for self-powered, highly sensitive 2D photodetectors.