How to calculate the standard Gibbs free energy of formation of bromine vapor at room temperature?
If you calculate the equilibrium vapor pressure at 298K (25 C), you can calculate the change in Gibbs free energy of the vapor in going from this pressure to the hypothetical state of 1 bar and 298K using $dG=VdP=RTd\ln P$.  Neglecting the Poynting correction, the free energy of the liquid at 298 and at the equilibrium vapor pressure is 0.  And this is also the free energy of the vapor at 298 and at the equilibrium vapor pressure.