Three things people should know:
The Gedo Mazo was NOT made by Hashirama.
The Gedo Mazo is likely made of Mokuton
The Gedo Mazo is likely only a summon of Madara's Rinnegan
How it was made:
After Madara's battle with Hashirama, Madara took his DNA and had the ability to use Mokuton. After the fight Madara made the plan to Tsukuyomi the world because of his failure to defeat Hashirama. To do this he knew that he needed immense chakra to constantly Tsukuyomi the entire world, so he made his plan to revive the Juubi. Madara then made the Gedo Mazo to seal tailed beasts and to be a vessel of the Juubi since the actual body was in the moon.
What happened after:
Around this time I'd think is when Madara started lacking ability to do things on his own, so it is likely that he gave his Rinnegan to Nagato around now. Then he acquired his one Sharingan and soon found Obito and now we're seeing what happened now...
Now after Obito acquires Madara's Rinnegan he gained the ability to use the Gedo Mazo.
tl;dr Madara made the Gedo Mazo to maintain the Juubi out of Mokuton since the actual body was in the moon.
The Gedo Mazo was NOT made by Hashirama.
The Gedo Mazo is likely made of Mokuton
The Gedo Mazo is likely only a summon of Madara's Rinnegan
How it was made:
After Madara's battle with Hashirama, Madara took his DNA and had the ability to use Mokuton. After the fight Madara made the plan to Tsukuyomi the world because of his failure to defeat Hashirama. To do this he knew that he needed immense chakra to constantly Tsukuyomi the entire world, so he made his plan to revive the Juubi. Madara then made the Gedo Mazo to seal tailed beasts and to be a vessel of the Juubi since the actual body was in the moon.
What happened after:
Around this time I'd think is when Madara started lacking ability to do things on his own, so it is likely that he gave his Rinnegan to Nagato around now. Then he acquired his one Sharingan and soon found Obito and now we're seeing what happened now...
Now after Obito acquires Madara's Rinnegan he gained the ability to use the Gedo Mazo.
tl;dr Madara made the Gedo Mazo to maintain the Juubi out of Mokuton since the actual body was in the moon.