Originally Posted by Grouchy

Learned that the hard way several years ago
She was right. Sadly no GUC will clear at the gate @AMS as per point 10 of the GLOBAL UPGRADE TERMS & CONDITIONS;
Airport standby is not available on Air France or KLM operated flights as well as Delta operated flights originating from AMS, BOM and CDG.

If you want to upgrade try the ticket office (landside)
Departure Hall 2
Opposite check-in desk 16 or one of the KLM lounges.

Strange however that your GUC was closed.