What does the TLS 1.2 client finished message contain?
rfc5246 7.4.9 defines 
verify_data as 

     PRF(master_secret, finished_label, Hash(handshake_messages))
        [0..verify_data_length-1];


Note the second line; this effectively truncates the PRF output to verify_data_length octets. It goes on to say that verify_data size 


  depends on the cipher
        suite.  Any cipher suite which does not explicitly specify
        verify_data_length has a verify_data_length equal to 12.  


ECDHE_RSA_AES_128_GCM_SHA256 is defined in rfc 5289 which does not explicitly specify, so this default applies.

Second, someone somewhere is wrong. GCM and CCM ciphersuites do not have a HMAC on the (plaintext) record, although stream and 'block' (CBC) suites do; instead an authentication tag is added to the encrypted record by the AEAD process. Compare 6.2.3.3 to 6.2.3.1 and 6.2.3.2.