Gd(Co1-xGax)2 (x = 0, 1/6, 1/3, 1/2, 2/3, 5/6, and 1) phases were synthesized by arc melting the constituent elements and subsequent annealing. The samples were characterized by powder and single crystal X-ray diffraction, magnetic measurements, and electronic structure calculations. An interesting structural sequence was obtained: cubic MgCu2-type structure for x = 0 and 1/6; MgZn2-type structure for x = 1/3; orthorhombic SrMgSi-type structure for x = ½; orthorhombic CeCu2-type structure for x = 2/3; hexagonal AlB2-type structure for x = 5/6 and 1. Tight-binding linear-muffin-tin orbital (TB-LMTO) calculations were performed on GdCo2, GdCoGa, and GdGa2 to trace the origin of their structural transformations, which appear to be driven by the changes in the valence electron count (VEC). In addition, some conclusions of these fully stoichiometric compounds were obtained from the rigid band model. GdGa2 (x = 1) is antiferromagnetic, while the other ones are either ferrimagnetic or ferromagnetic. TC of Gd(Co1-xGax)2 decreases monotonically with the increasing Ga content, suggesting that promising room temperature (RT) magnetocaloric materials could be obtained between x = 1/6 and x = 1/3.