Since last friday all the that is forwarded from my mail server to a Gmail address get's spam at Gmail. Did Gmail change anything? I'm not on any blacklist as far as I can see....
Try to create a SPF records of domain name who you forwarding to Gmail. Create a TXT record just of domain: v=spf1 a a:HostnameWithPTRRecord ip4:YourRealIPOfServer -all