In the introduction of NIST Special Publication 800-38D is said that:
GCM provides assurance of the confidentiality of data using a variation of the Counter mode of operation for encryption. GCM provides assurance of the authenticity of the confidential data (up to about 64 gigabytes per invocation) using a universal hash function that is defined over a binary Galois (i.e., finite) field. GCM can also provide authentication assurance for additional data (of practically unlimited length per invocation) that is not encrypted.
As a block cipher GCM guarantees the confidentiality of data, but what does it mean that it assures its authenticity? And what is exactly the authentication assurance for the unencrypted additional data?
I believe that data authenticity may correspond to its integrity, i.e. the absence of tamperings, while I'm not sure of the meaning of the authentication for the additional data.