This
http://www.amctv.com/breaking-bad/vi...-412-end-times interview thats up on AMC with cranston and giancarlo esposito seems to suggest that Gus was not aware of the poisoning and that as he was walking to his car he had an epiphany.
Key parts start at the 3:27 min mark
from real life giancarlo:
" Gus doesnt really know the circumstance of all that happened. Then finds out what that is."
"then a clue happens." (when jessie says boy was poisoined)
"as hes walking out hes thinking in his head, the boy was poisoned? poisoned? poisoned?"
"and he realizes he parks where hed always park. Then it hits him."
While this doesnt confirm that Walt did it (ive read theories that Saul set it all up), I think it does kinda remove Gus from the List of Possible people
I also dont think its possible the boy ingested it accidentally based off the time frame that jessie gave for when he last had the cigs and bc its not like ricin was mixed in and could be smoked. It was actually stored in a vial inside the cig. I really want Walt to be behind all of this bc I think it just shows how terrible hes become, but idk if thats for sure what the writers are doing.
ANother thing in support of walt doing it though is we have to remember how big of an ego Walt has. Even in EP 1 of this season when jessie and walt have the sit down in front of Mike and Gus and its really tense Walts ego is huge. Same thing with Hank at the dinner table and cant let Gail take the blame as heisenberg. So why would Walt, actually admit to ever being outsmarted by Gus. That doesnt fit with his whole ego thing, even in a desperate spot, unless he was lying.