This is probably the yolk sac which sometimes remains attached. Keep the floor of the gecko's cage moist. The yolk sac will dry up and fall off soon on its own and it will be fine. The only other thing to watch for is to make sure that there isn't an opening in the gecko's belly. Sometimes the belly doesn't close up. The gecko will probably not survive in this case. It's happened to me twice in 9 years. For now, assume it's just the yolk sac (since there's nothing you can do in the other case) and treat it as I described above.