For biochemical reactions, this property refers to the standard transformed Gibbs energy change for a reaction written in terms of biochemical reactants (sums of species), delta-G'o. delta-G'o = -RT lnK' and delta-G'o = delta-H'o - T delta-S'o delta-G'o has units of kJ/mol. Like K', it is a function of temperature (T), ionic strength (I), pH, and pMg (pMg = -log10[Mg2+]). Therefore, these quantities must be specified, and values for DELTA-G for biochemical reactions are represented as 5-tuples of the form (delta-G'o T I pH pMg). This property may have multiple values, representing different measurements for delta-G'o obtained under the different experimental conditions listed in the 5-tuple. (This definition from EcoCyc)
Source:http://www.biopax.org/release/biopax-level2.owl#DELTA-G