I did some quick calculations based off the given dimensions of the bags. Since Zpacks is basically an envelope and GG uses more of a box shape, for fabric area I get:
GG
.33sqft @ standard .74oz/sqft cuben
Zpacks
.31sqft @ heavier 1.0oz/sqft cuben
So with some unit conversions and dimensional analysis we get an estimation of:
7.02 grams fabric for GG and 8.73 grams fabric for Zpacks. Take into account same buckle weight, thread weighs more than tape, and addition of velcro it's not hard for the 1.7g difference to grow to about 3grams.
That said we all just put in a lot of effort over 3 grams. For perspective if you forgot to take a penny out of your pocket on a hike, that's 2.5 grams of added weight…