Results obtained assuming that GA forms exclusively in the gas phase via the ethanol tree. Model predictions (curves) are compared with observations (symbols) toward B0, B1 and B2, assuming three excitation temperatures T_ex = 15 K (crosses), 30 K (triangles) and 50 K (squares) . In each figure, the upper panels show the abundances of GA (blue) and ethanol (red) as a function of time. The bottom panels show the evolution with time of the GA/ethanol abundance ratio. Left panel: predictions obtained assuming a temperature of 100 K and four values for the injected H₂O abundances: 1.0 (dotted), 2.5 (solid), 5.0 (dashed) and 10 (dotted-dashed) ×10⁻⁵). Right panel: predictions obtained assuming an injected H₂O abundance of 2.5 × 10⁻⁵ and four temperature values: 80 K (dotted), 90 K (dashed), 100 K (solid) and 130 K (dotted-dashed).