A systematic study on the formation and magnetic properties of Tsai-type 2/1 approximants in Ga–(Pd, Pt)–Tb alloys was conducted by replacing Pd with Pt in a previously reported antiferromagnetic Ga–Pd–Tb 2/1 approximant. Tsai-type 2/1 approximants were found to form quaternary Ga–(Pd, Pt)–Tb alloys as well as ternary Ga–Pt–Tb alloys. Magnetic susceptibility measurements indicated that the substitution of Pt for Pd in the 2/1 approximants resulted in a change in the magnetism from antiferromagnetic to spin-glass-like behavior.