Thanks for point the mistake in the reply. If the signals are zero mean The Power cluster is very close to the GFP.^2 or the variance across sensors. RMS in fact is the one that is similar to GFP.
This is true in Brainstorm as GFS is computed with std(x,1,1), where the first 1 indicates that the standard deviation is normalized with N not with N-1
This was mentioned also in a previous post:
Global Field Power Calculation Difference
P.S. I checked my notes, and indeed, "variance" is the term I was comparing with power.