Gmail does not accept mail from a new server

Here is the text from google's help page:

Additional guidelines for IPv6

• The sending IP must have a PTR record (i.e., a reverse DNS of the sending IP) and it should match the IP obtained via the forward DNS resolution of the hostname specified in the PTR record. Otherwise, mail will be marked as spam or possibly rejected.
• The sending domain should pass either SPF check or DKIM check. Otherwise, mail might be marked as spam.

So, yes, if you use IPv6 to deliver your mail to Google, you MUST have a PTR matching the sending's system IPv6 address.

If you really can't create a PTR for your IPv6 address (which seems weird to me), the solution is to either disable IPv6 completely or to setup your main server to prefer IPv4 for outgoing connections. Don't forget to change the SPF record as well if you do that.