We rarely have more than 30-40k in the gbank in terms of raw gold. I don't think hoarding more than that is truly beneficial. Keeping mats, and various other popular tid-bits as rewards for progressing worked a lot better for us. I tend to view the gbank as the soup kitchen of our guild community. When we're on hard times, it's there to cushion the impact of long-nights wiping, a bit of a push to craft new gear, and helps encourage players to stay interested in the raid content rather than how much gold is on their main.