not used the g&g but i'd wager the quality is better on it, plus unlike the agm it has a quick swap barrel (although maybe of limited utility in airsoft but still cool) but for pretty much triple the price you'd expect it to be much better.
i have the agm and whilst it's an imposing gun the materials are a bit lackluster in the reciever, i've fixed it by bolting in 2 big bars of steel on the inside which doesn't help the weight, the finish is just black paint which leaves shiny aluminium when it inevitable chips off.
the bipod is a bit annoying and floppy to work with, however it is at least pretty strong unlike on some other types like the m60.