So the gedo mazo could actually be created by hashirama to control the bijus (all of this is not confirmed yet) as it seems as it is made from wood. however we have seen every character that possesed rinnengan being able to summon it exept madara but it seems normal to me that he could since he basically owned it after hashiramas death. moreover if youthink about it if madara really gave his eyes to nagato then obito nagato and madara all of them possesed madaras eye's (yea even obito cause he took nagato's eyes which if madara gave his to nagato that makes the eyes obito took madara's eyes) so maybe there is a special contract to the gedo mazo summon and madaras eyes i do not think its the best theory but i just wanted to share it .
1 thing i would like to throw into this pool of speculations is this. there might be multiple gedo mazo's.
-first of all pain summoned the gedo when his friend died during the flash back in the conflict against the salamander (guy who named jiraya's group the legendary sanin) and used it to kill those ninjas who were with the salamander.
-second the speculation that the gedo from the recent chapters of the manga might be madara's experimentation with harashima's body which may have resulted in the rule that ninjas be burned when they fell during missions so that they do not expose secrets of their clan/village/mission. because compare to the gedo pain summoned it is larger hence being a completely different gedo.
-third the gedo does not actually control bijuu's it suppresses them because harashima was never shown to control the 9 tails all we saw was his ability to restrain it and release it from madara's control. meaning that the sharingan is the only known tool to actually control bijuu's against their will.
-fourth is the fact that it is believed that the gedo that obito uses may actually be the shell of the 10tails. if this is true i believe that it is more like a compiler to properly compress all the bijuu's .so how does harashima's body/cells/(insert w/e shit that makes sense to you here) factor into the fact that it was artificially created.
what i do know is
-how obito survived and what he had been doing all these years
-the fact that his body melted ties directly with manga ch.603 since he had an artificial body just like zetsu(white / black) after taking severe damage from the rasengan.
-the spiral nature of his mask also now makes sense being covered by a zetsu.
now my question is shouldn't obito have been smaller since during that era(16+yrs ago when the kyuubi attacked konoha) they were kids(his ninja group obito+rin+kakashi)?
why not go after kakshi to recover his other sharingan to awaken his susanoo?
why waste the chance he had when he obtained the kyuubi all those years ago when he could've had it all those years ago and be done with it ?
when did the real madara die cause it looks like he set all these things in motion for his revival and why is his edo tensei form much younger than what the manga ch 602 reveals?