Dunno if there's some carve-out for GRU, but the AMEX site has this to say under "Who is eligible for complimentary access to The Centurion® Network?"
https://thecenturionlounge.com/info/access/
Delta SkyMiles® Reserve Card Members are only eligible to visit The Centurion® Lounge when flying on a same-day Delta-marketed or Delta-operated flight and eligible lounges only include all Centurion Lounges located in the United States, Hong Kong, and London.