The binary GdNi adopting the orthorhombic CrB-type (oS8, Cmcm) dissolves up to 21 at% Co (GdCo_0.42Ni_0.58), but when the Co content reaches 23.7 at% the crystal structure of the GdCo_xNi_1−x phase changes from the orthorhombic CrB-type to the tetragonal MoB-type (tI16, I4₁/amd). The latter remains stable between 23.7 and 36.0 at% Co (0.47 ≤ x_Co ≤ 0.72), and there is a two-phase region compositional gap of ≈ 2.5 at% Co between the two structures. One of the two 8e sites available in the MoB-type structure is filled by Gd atoms, while Co and Ni atoms are randomly occupying the other 8e site. The lattice parameter a increases from 3.9485(1) Å to 3.9591(1) Å, while c decreases from 21.3286(1) Å to 21.2404(1) Å as x_Co increases, both following a quadratic trend. As a result, the unit cell volume increases from 332.53(2) ų to 332.93(2) ų for, respectively, the Co-poor (GdCo_0.47Ni_0.53) and the Co-rich (GdCo_0.72Ni_0.28) sides of the solid solution, also following a quadratic trend with a negative deviation of the Zen’s law. The title compound becomes ferromagnetic with T_C increasing linearly from 150 K for GdCo_0.50Ni_0.50 to 171 K for GdCo_0.70Ni_0.30; linear interpolation leads to T_C = 127 K and 174 K for the two ends of the solid solution with x_Co = 0.47 and 0.72, respectively. The magnetic susceptibility follows the Curie-Weiss law, indicating a trivalent state of Gd ions, with an effective magnetic moment slightly increasing with the Co content. The magnetocaloric effect, calculated in terms of the isothermal magnetic entropy change, decreases slightly as x_Co increases, remaining large in the range 127–174 K.