We have recently published a paper proposing a tissue correction that accounts for voxel segmentation, both in terms of water relaxation/visibility, and the different GABA concentration in gray matter and white matter.
This paper can be found here.

The different corrections proposed are now implemented in Gannet, with output names that are not entirely clear... so, here are the equivalencies.

MRS_struct.out.GABAconciu
This is the uncorrected GABA concentration (relative to water).

MRS_struct.out.GABAconciuTissCorr
This is the CSF-corrected GABA concentration (relative to water). (We should probably change the naming !). It is MRS_struct.out.GABAconciu divided by (f_GM+f_WM), and is filled by GannetSegment.

MRS_struct.Quantify.QuantTissCorrGABA_iu
This is the GABA concentration (relative to water), corrected for the visibility and relaxation of the water signals in GM/WM/CSF and for the GABA concentration differential between GM and WM (alpha in the paper). It corresponds to Equation 5 in the paper. It is filled by GannetQuantify.

MRS_struct.Quantify.QuantNormTissCorrGABA_iu
This is the most fully corrected GABA concentration (relative to water), corresponding to Equation 6 in the paper. It is filled by GannetQuantify. This is what we recommend as the measure to be used.