how do you think he summoned Kurama to battle Hashirama?
Jesus. I didn't think I would have to explain this much.
A summoning contract is a mutual understanding between two parties, in which the summoner sacrifices blood and chakra in order to summon the summonee for a small amount of time.
This contract in no way gives the user direct control over the summon. Examples of this are every ****ing summon in the entirety of the manga.
The Sharingan and Rinnegan however, give direct control over their summons, with simple eye contact being enough to last decades.
There is no contract for the Gedo Mazo
There is no contract for the Rinnegan Animals
There is no contract for the Kyuubi.
These summons are all treated as tools, and are summoned no differently than if they were Kunai from Sasuke's wrist guards.
Genjutsu them and then do the contract. Why do you think obito was incapable of summoning the kyuubi after minato put the contract seal on him? You can't just genjutsu something and can automatically summon it :|
The Rinnegan says differently.