Stripping off message authentication or signature If attackers can strip off RSA / EC / -DSA digital signature and conduct CCA on AES-CTR or CBC payload, why can't they do the same for AES-GCM?
If attackers can strip off RSA / EC / -DSA digital signature and
  conduct CCA on AES-CTR or CBC payload, why can't they do the same for
  AES-GCM?


The scenario, you're talking about is iMessage or Signal Protocol or other protocols which allow optionally to sign the ciphertext and thereby don't MAC it.

The problem here is a) that you could replace the signature with your own (if it is valid and required) or b) completely strip it off if it is not required, which would be considered a protocol flaw in my eyes.

The situation is different with AES-GCM because the authentication is a mandatory part of the specification. Any non-stupid implementation will return an "invalid ciphertext" if the tag is missing or just the last ciphertext block (i.e. largely unrelated), so this won't allow you to even get to decrypt the CTR stream and thereby exploit its CCA weaknesses.