I've never seen a GG dealer in my life so I'm in the same boat as you! Motocrosscenter.com has the best prices I've come across by a large margin if you don't mind ordering from overseas. I've been hesitant to place my order since a few of the small parts I need are out of stock and I don't know the current situation on backordered GG parts and don't want to pay separate international postage for a couple tiny pieces. They do show their current inventory right on the site which is convenient and most of what I need for my '01 300 shows in stock.
In the US, I've ordered LOTS of parts through Hall's cycle (all Husqvarna parts, but they do sell GG), some from Matto Cycle, and some from Afterhours Cycle. I too would be interested to know if there is an online/US retailer with a streamlined ordering process (shows inventory and able to "add to cart" instead of placing an order and later finding out some of your parts are not available/on backorder.)