My understanding is that to affect GCOR during preprocessing, you need to add the GM as a 1st level covariate prior to denoising.  I've been wondering about what the QA_GCOR would do as a 2nd level covariate, as I would like to control for potential global signal differences between conditions/timepoints without actually doing global signal regression at the 1st level.  I don't think they are the same thing.
 
Jeff