I believe is a shell of the ten tail.when all the tail are collected then it will turn fully to ten tail ,similar to susano having different level of development.
It's never been really specified but it seems to be a statue summoning of the Rinnegan that needs a Chakra source to be synchronized to by either filling it with chakra like with the Bijus or impaling black chakra transmitting rods like it did on Nagato
Some people believe it's the body of the 10 tails and that's why the tailed beast are been sealed inside it and it's able to use their power
in the flashbacks with the s06p we have only seen the outline of the juubi
gedo mazo looks exactly like that outline, and also i think its the juubi because only people with the rinnengan can summon it and i think it was in the moon's core but then it was summoned to earth by nagato to fight hanzo
Tobi already said the SO6P sent the Juubi's body into the moon with Chibaku Tensei. Gedo Mazo is just a summoning of the Rinnegan that can store Tailed Beast into it. How does it look like the outline of the 10 tails?