TLS 1.2 Cipher Suites With AES-GCM – What data (if any) is passed to the AES-GCM cipher as the Additional Authentication Data?
Actually section 6.2.3.3 of RFC 5246 talks about the associated data:


  The additional authenticated data, which we denote as
  additional_data, is defined as follows:

additional_data = seq_num + TLSCompressed.type +
                    TLSCompressed.version + TLSCompressed.length;

  
  where "+" denotes concatenation.


So the sequence number, the packet version, the packet type and the packet length are passed as associated data to the cipher.



So why is exactly this data authenticated?


The sequence number: Authenticating this helps prevent re-ordering and replay attacks and binds the sequence number to the data packet.
The protocol version: Authenticating this ensures that if you can't downgrade the protocol version (eg to SSLv3) to exploit weaknesses and learn something about the content of the current packet.
The packet type: I suppose this blocks you from re-using a handshake packet for an application traffic packet (which may have independent sequence number streams)
The packet length: This blocks you from silently chopping some data off somewhere, but is mostly here because it's public data and authenticating it doesn't really hurt.