Just spoke with AMEX rep via chat.
I just got a PRG and used it to purchase the $100 GC 2 days ago. Noticed my new statement closes on 1/XX/2016 so was a bit leery if I would get my credit for 2015.
Anyhow, he said not to worry it is based of transaction date and I should have no issue receiving the credit for 2015 even if the statement closes in 2016.
Fingers crossed.