The things we avoided putting in the gray tank were grease, food scraps and coffee grounds. We wipe the plates/pots quickly wipe our used napkins or used paper towels before they go in the dish water.
If you are one to hook up the sewer hose and leave your gray water valve open all the time you'll be able to see the greasy sludge left behind when you peek in that sewer hose. That same sludge would be in the bottom of your tank. Gray tanks can smell worse than black.