The G-factors option defines how the G-factors for the main-chain bond lengths and angles are to be computed.
The option is:-
G-factors --------- Y <- Use Engh & Huber means for bond length/angle G-factors(Y/N)?
By default these G-factors are computed using the Engh & Huber (1991) small-molecule means and standard deviations. Where a structure has been refined using a different dictionary of values for these parameters it may appear to have very "unusual" geometry according to the PROCHECK G-factors.
Thus, if this option is set to N, the programs compute the G-factors on the basis of the observed mean and standard deviation values in the given structure. This still allows any significant outliers to be highlighted despite a different dictionary having been used.