For what do I need the Guild Treasury in GvG?
As your guild conquers and expands its territory, goods cost rise. Your whole guild may need to contribute goods to pay for new army deployment and unlock defense slots, and this is where the guild treasury comes in. When you attempt to place an army, you are first taken to the unlock screen, where you can contribute to - your guild's treasury.
In the image below, 30 iron and cloth are required, and as you can see that they are already available in your guild treasury, because the number is shown in green. Also needed are ebony, jewelry and limestone, which the guild need to collect and pay in order to unlock this slot. Buttons beside the good requirement show what you as a player are able to contribute: in this case, 15 jewelry and 15 limestone. And you could also use Diamonds to purchase ebony for your guild. Only when all required goods are available can the slot be unlocked.HINT:
Granting freedom to a sector your guild no longer wants will also result in goods being paid to your guild's treasury
You can also donate goods to your guild's treasury whenever you feel like it in the Treasury screen.