Magnetic tunnel junctions (MTJs) have been widely applied in spintronic devices for efficient spin detection through the imbalance of spin polarization at the Fermi level. The van der Waals (vdW) nature of two-dimensional (2D) magnets with atomic-scale flat surfaces and negligible surface roughness greatly facilitates the development of MTJs, yet is only restricted to ferromagnets. Here, we report A-type antiferromagnetism in 2D vdW single-crystal (Fe0.8Co0.2)3GaTe2 with TN~203 K in bulk and ~185 K in 9-nm nanosheets. The metallic nature and out-of-plane magnetic anisotropy make it a suitable candidate for MTJ electrodes. By constructing heterostructures based on (Fe0.8Co0.2)3GaTe2/WSe2/Fe3GaTe2, we obtain a large tunneling magnetoresistance (TMR) ratio of 180% at low temperature and the TMR retains at near-room temperature 280 K. Moreover, the TMR is tunable by the electric field down to 1 mV, implying the potential in energy-efficient spintronic devices. Our work provides new opportunities for 2D antiferromagnetic spintronics and quantum devices.