An assembly includes a gearbox for a gas turbine having a geartrain, and at least one starter/generator coupled mechanically to the gearbox. The starter/generator includes a generator module with a first casing, a generator housed in the first casing, and a first shaft constrained to rotate with the rotor of the generator, projecting from the first casing, and carrying a first mechanical coupling member; an exciter module, including a second casing, an exciter housed in the second casing, and a second shaft distinct from the first shaft, constrained to rotate with the rotor of the exciter, projecting from the second casing, and carrying a second mechanical coupling member; and an electrical connection including a rectifier and at least one connector for connecting the secondary circuit of the exciter to the primary circuit of the generator.