We report the discovery of a new solid solution Gd-2(Co3-xSix) with 0.29 < x < 0.50 in the Gd-Co-Si ternary system. Members of this solid solution crystallize with the La2Ni3-type structure and correspond to the stabilization of "Gd2Co3" through silicon substitution. The structure of the member Gd-2(Co2.53(3)Si0.47) was determined by X-ray diffraction on a single crystal. It crystallizes with the space group Cmce and cell parameters a = 5.3833(4), b = 9.5535(6), and c = 7.1233(5) angstrom. Co/Si mixing is observed on two crystallographic positions. All compounds studied in the solid solution present a ferrimagnetic order with a strong composition-dependent Curie temperature T-c with 280 K < T-c < 338 K. The magnetocaloric effect, which amounts to around 1.7 J kg(-1) for Delta H = 2 T, is interestingly tunable around room temperature over a temperature span of 60 K through only 4-5% of composition change.