Vgs is the gate-souce difference for a MOSFET (or some
numner of, in parallel.
Vgg would be a single ended supply, ground referred.
-If- all FETs' sources are attached to ground then
"Vgs" and "Vgg" might be the same electrically, but
clarity-of-purpose prefers Vgg as a supply net name.
Maybe some Vgg supplies are noncritical and don't
need to be -exactly- the "typical" value. But for PAE
you wouldn't want any more voltage on the final, than
it takes to accommodate signal swing (& interests such
as clipping / distortion / IP3).