G2G means Grain to Grain transfers.
Once you have learned to prepare sterilized spawn pint or quart jars of bird seed, rye or grains & fully colonize them with mycelium. You can easily propagate a single colonized quart jar of that into about 20 more via G2G transfers. Spawn rates ranging from 1:10 to 1:40 are being used, with 1:40 being "on the verge of being unsuccessful" [Stamets, GGMM 2000].
Jars propagated via G2G transfers generally colonize under optimal conditions 100% in 10 to 14 days . Given that, spores do not have to germinate, as what is transferred to freshly prepared jars is active mycelium on fully colonized seed or grains.
The method is to prepare fresh jars, just as you would to inoculate via a spore syringe (soak seed, drain, load, apply filter disk & PC). Except, rather than inoculate the fresh jars with a syringe, you transfer grain from a colonized jar to fresh uncolonized jars. The procedure is simple & only requires common sense, minimal preparation, and the cleanest hygiene you can muster. A glove box is highly recommended and a basic one is not any harder to do than cutting holes in a clear tote and flipping it upside down.
Best case scenario you have a flowhood, next best is build a simple Still Air Box (SAB) to GTG inside. Otherwise prepare the smallest cleanest uncarpeted room you have (generally, a bathroom). In the following manner. Clean the room as best you can, getting rid of any dirt, dust, mold or mildew. Remove any cloth hanging anywhere. Spray Lysol on everything, everywhere & wipe it down. If you have any HEPA type air filter unit, place it in the room & run it for at least 1 hour. Running a HEPA is preferable but if you don't have one, you can usually manage without it.
Wipe your fully colonized jar of grain & your fresh jars down, with a clean Lysol or rubbing alcohol sprayed paper towel. Place those in the room, on the counter top, or whatever flat working surface you intend to use. Wear freshly laundered clean cloths. If you have a face mask (preferable) wear it. If you have a shower cap to cover your hair (preferable), wear it. Enter the room, spray Lysol around (again) run the HEPA for a few minutes (if you have one). Then, turn it off. Spray your hands & arms with Lysol & wipe dry.
Shake up the colonized jar breaking all the grains loose. Avoid using your hand to beat the jar against. Some good things to use are a 5 gal pail, a tire, or any relatively soft yet firm item. In the event you cannot break the grains loose use a sterile spoon to break them loose. Sterilize the spoon in your pressure cooker or oven wrapped in foil. Once the grain is broken up crack the lid and all the lids you plan to transfer into. This will avoid any struggles when/if you encounter a tight lid mid stroke on your GTG's. When ready open the master jar avoiding touching the lip, remove the first fresh jars lid and start to pour a small amount into each jar. Once each jar gets its grain loosely place the lid on and move to removing the next lid. The master jar should stay in your other hand the whole time if all goes well. One jar can inoculate 10-20 jars if you're careful about portioning.
Repeat this same process as many times as you have fresh jars to transfer to. Once done shake each fresh jar to spread the colonized material throughout it. Place your fresh jars in a dry, dark, warm place (preferably between 70-80F), and allow them to colonize in peace & quiet. G2G transfer & shaking jars batters the mycelium. It takes it a 2-3 days to recuperate from that shock. There is no need to shake G2G jars more than once. As, doing so will only slow colonization, rather than speed it up.