Mirror symmetry of transmission is generally considered an inherent property of reciprocal systems. Metagratings, governed by the acoustic diffraction laws in periodic lattice, have brought new vitality to classical wave physics and its manipulation. However, even using the diffraction ability of waves, it remains a challenge to break the mirror symmetry of wave transmission through a single passive metagrating, while simultaneously controlling the wavefront in acoustics. Here, we propose a widely applicable description of acoustic wave diffraction in metagratings by incorporating the coupling between transmitted and reflected waves. Through deliberate design of the transmission and reflection phase gradients, we construct a monolayer passive phase-gradient metagrating (PGM) that offers advantages of structural integrity and ease of fabrication. This design enables the breaking of mirror symmetry in both acoustic planer wave transmission and reflection within a reciprocal system. We further demonstrate its applicability across a broad range of incident angles, facilitating versatile wavefront manipulation. Our work provides a pathway to advance acoustic communication and isolation, and the underlying principle can be extended to other classical wave systems.