Okay, I understand what you're getting at. I similarly dislike that and any other phrase being used to mock ABDLs, but if someone within our community uses those nonsense words as part of their ageplay, I don't have any problem with it. I think that it doesn't have to be correct to feel right for one's little side.
After a cursory Googling, I can't find anything on the etymology for 'Goo goo gaa gaa', which suggests that someone just made it up and everyone was happy to go along with the usage of this unrealistic word to connote 'baby noise'! Even Dr. Phil used 'Goo goo gaa gaa' in a question during his episode with Baby Brett.