Hello,
One thing that can give you apparent camera rotation problems is the character rotation settings. Please check if your character’s settings match these pics:
Another probable source of problems in your case is the fact that your arms are a separate mesh from the body and you’re trying to use the GSA full-body character. The GSA full-body character has the camera attached to the head bone in the same mesh of the arms (this is how it knows where the camera is on order to align the sights with it) and it also bends the spine/neck bones in the same mesh of the arms when you look up and down. For things to work properly in your case, you would need to make sure that the code still knows where the camera is and also in which mesh to rotate the spine bones when you look up and down.