Simulated Gaia astrometric data of the KELT-20 host star for best-fit brown dwarf and stellar-companion solutions. Blue and black points are mock DR4 and DR5 measurements. Gray curves indicate the underlying model reflex motion. The origin is the center of mass. Left: expected DR4 and DR5 orbital coverage of the host star under the perturbation of a brown dwarf companion (m₁ = 56 M_J, a₁ = 6.3 au, e₁ = 0.82). Right: expected orbital coverage in the presence of a stellar companion (m₁ = 0.17 M_⊙, a₁ = 11 au, e₁ = 0.94).