Atom interferometry offers new perspectives for geophysics and inertial sensing. We present the industrial prototype of a new type of quantum-based instrument: a compact, transportable, differential quantum gravimeter capable of measuring simultaneously the absolute values of both gravitational acceleration, g, and its vertical gradient, Γzz. While the sensitivity to g is competitive with the best industrial gravimeters, the sensitivity on Γzz reaches the limit set by quantum projection noise-leading to an unprecedented long-term stability of 0.1 E (1 E=1×10 −9 s −2 ). This unique, dual-purpose instrument, paves the way for new applications in geophysics, civil engineering, and gravity-aided navigation, where accurate mapping of the gravitational field plays an important role.