Thanks for your reply.
>Are you running a static IPV6 IP address?
>Did you replicate all of the ports on IPV6?
Yes all 6 services enabled at Settings > Bindings > IP Addresses > Respective IPv6
>Did you map the new IPV6 ports to the PUBLIC IPV6 IP address - must NOT be a private or natted IPV6 address, but a PUBLIC IPV6 address,
I don't know what you mean by mapping, I know the port configuration done as i mentioned above. I don't have any NAT, nothing special configuration, it's just same as IPv4 settings in Windows. I can ping between two different servers and google public DNS via IPv6
> Did you add a HOST A RECORD with the IPV6 IP address?
Yes, AAAA record created
> Did you add the new IPV6 HOST A RECORD to the domain's MX records?
I have only A record, mx pointing the server's IPv4.
> Did you add the IPV6 IP address do DNS, rDNS., SPF, and DMARCIAN
rDNS ok, did lookup and got result OK.
SPF added in DNS zone settings which is stored in cPanel DNS. It's same as IPv4 with a little difference; "ipv4" replaced with "ipv6" in the txt line in cpanel's DNS zone.
DMARCIAN i don't know this, i will check.
Result: Gmail still shows only server's IPv4, regardless I did set "Unassigned" or assign "Server IP" in Domain outbound IP v4 > Technical TAB.