I think Gamamaru is the Hogoromo's brother bcoz hes not mentioned to be dead so i think he's alive so it must be him.And due to more intake of more energy a person could become frog as we see when Naruto training for sage mode and you can see during pain arc too one energy absorber pain turns to frog thats what happened to Hogoromos brother.Sorry guys if my prediction is wrong.
'
'