You can’t. When my ISP switched me to CGNAT, I spent days upgrading everything to IPv6, only to discover that gmail didn’t even support it! (Mail Server to mail server, not the web app) I gave up, asked my ISP IPv4 back and, fortunately, got back a new IPv4. But I fear the day that option will disappear…
What year was this? While I can't find a source I believe Gmail has supported IPv6 for sending and receiving since the World IPv6 day back in 2011. I've certainly been doing it since 2017.
Your issue might be rather that Gmail actually enforces all their guidelines on IPv6 instead of silently degrading your reputation behind the scenes like they do for IPv4. So proper RDNS, SPF and DKIM are tablestakes with DMARC and MTA-STS strongly recommended.