J'ai posé la question au support Gigabyte, et d'après eux la mémoire ECC est totalement supportée, Voici la réponse reçue (si ça peut aider d'autres gens qui se posaient la question) :
"Dear customer,
Update from our eam:
User’s CPU 3900X supports ECC memory running in ECC mode.
Thank you for emailing GIGABYTE.
We would like to help you on this issue.
We are looking into the issue and will come back to you ASAP after figuring it out clearly.
Thanks for your patience.
Regards
GIGABYTE Team"