Waterfowl grower pellet. Beware of using crumb as it gets too sticky with all the water they constantly dibble into and can bung up under their tongue. Goslings are generally big enough from hatch to cope with pelleted food. It's important they do have a proper waterfowl pellet and not chicken pellet as they can develop a condition called angel wing (particularly the males) from the high protein content in chicken pellet.
Are they being hand reared or reared by broody goose?