The documentation ("The Libgcrypt Reference Manual -Version 1.10.1") mentions that the GCM-SIV mode is implemented according to the "RFC-5297 (AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption)":
However, I think it should rather be: RFC-8452 (AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption).
The reason is that the RFC-5297 discusses the generic SIV mode, while RFC-8452 discusses the actual (AES)-GCM-SIV mode.

The corresponding section can be found in section 5.2 Available cipher modes for the cipher mode GCRY_CIPHER_MODE_GCM_SIV .