The negative of the Gibbs energy (ΔGr°) change for the reaction:
G02587.png
in the gas phase.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1116 (https://doi.org/10.1351/pac199466051077)