Tom,
The solution is easy, just have a dual-stack MX record.
$ host gmail-smtp-in.l.google.com.
gmail-smtp-in.l.google.com has address 172.253.115.26
gmail-smtp-in.l.google.com has IPv6 address 2607:f8b0:4004:c06::1a
Servers using IPv6 connect to IPv6 as needed.
Matthew