I don’t know for sure what I would actually do.
I have two impulses – I want to protect both the child, and I don’t want the gorilla killed.
I have a notion that I’d try to climb down and distract the gorilla from the child somehow. But I also get that that’s partly silly and could get me killed or injured or do more harm than good.
On reflection, I think I would probably think about going in but not, and instead drive the yelping humans away from the scene. The gorilla was panicking because there was a crowd of human spectators it couldn’t really see who were suddenly making extreme panic sounds in response to every move it made. So I’d do all I could to drive the people away and get them to stop making all that noise. I think that’s the best way to try to protect the child from gorilla panic and the gorilla from human panic (and shooting).