Several other greylist filters (Postgrey, the sendmail milter, maybe more) are adding this header to the mail they process. Here is what the sendmail milter does (from their web site):
"Each message that went through the MTA has a X-Greylist header line, which report what milter-greylist did: message accepted immediately because the sender IP is white-listed, or because the recipient is white-listed. And if the message has been delayed, milter-greylist will report how long it had to wait because of greylisting before been accepted."
Would you be interested in added this feature to gps? I'm not sure that I agree with handing out whitelist information, a simple "delayed for nnn seconds" would be fine with me. Presumably, nnn would be zero for whitelist cases :-)
Thanks -- Steve