This is what im using to add comments to youtube :
gapi.client.youtube.commentThreads.insert({
part: \"snippet\", commentData
}).then(function
Insufficient Permission: Request had insufficient authentication scopes.
Means exactly that the currently authenticated user has not granted you the permissions to do that.
If you check the documentation comments.insert you will see that in order to use this method you must have authncated your user with the https://www.googleapis.com/auth/youtube.force-ssl
scope