G2G is grain to grain. One gets a pure fully grown jar or bag of whole grain and puts a few kernels into a new, sterilized jar or bag of grain as a starter or "spawn". g2g takes some practice and needs to be very sterile but it works to speed up your next grow. I don't use it but the whole point of this sort of grow or any as a matter of fact, is to multiply your mass of mycelium as much as you can. the more mycelium you hve the more mushrooms you will get. The only problem is sencience. If you g2g to a jar and then use that jar to g2g another, and do that a few times, your mycelium will become weak, fail to yield spores, grow misapen mushrooms, yield fewer mushrooms and finally the entire colony will die.
Whereas, if you take your colonized grain all from your initial jar or grow, you retard this process. Multiple generations will always cause sencience.