-- shouldn't be the Gmaill's IP range (64.233.160.0/19?) wwhitelisted?
Or does SpamCop just blacklist its competition (Gmail provides mail
services and spam filtering for free)?

This seems to be a long lasting problem:

http://chris-linfoot.net/d6plinks/CWLT-6CALWG

SpamCop lists GmailWednesday, 11. May 2005Category : GMailOh dear.
SpamCop is currently listing 64.233.162.202 (and possibly some others
close by) in its DNSBL. *

whois -h whois.arin.net 64.233.162.202 ...

OrgName:    Google Inc.
OrgID:      GOGL
Address:    1600 Amphitheatre Parkway
City:       Mountain View
StateProv:  CA
PostalCode: 94043
Country:    US

etc etc and

nslookup 64.233.162.202
Canonical name: zproxy.gmail.com
Addresses:
  64.233.162.202

According to SpamCop, "SpamCop users have reported [this] system as a
source of spam less than 10 times in the past week" and "In the past
13.8 days, it has been listed 4 times for a total of 26 hours".

So, SpamCop users (I count myself among their number) are allegedly
seeing plentiful spam from Gmail IPs, well enough to cause DNSBL
listing anyway.

The thing is, despite the many thousands of spam samples trapped here,
I still have only one sample of a spam ever originating in Gmail,
originating at rproxy.gmail.com ([64.233.170.203]) - I reported it.

So where's mine? I want some too.

Or is there another explanation?

SpamCop recently begun implementing a new algorithm intended to
minimise false listings by verifying bona fide mail hosts used by or
belonging to SpamCop users. I have registered my own mail hosts there
as well as the Gmail hosts so that if I receive a spam at my Gmail
account, I can report via SpamCop which I sometimes do. So possibly:

Some SpamCop users have attempted to configure Gmail mailhosts and got
it wrong, or

Some spammers have started to create convincing Gmail header forgeries
and are spamming SpamCop users who have not yet implemented SpamCop
mailhosts, or

Gmail has added new mail hosts that are not known to the SpamCop
mailhosts system

... or there really could be a Gspam problem...
Frankly I just don't buy that last one though. Could someone (anyone)
show me a real Gspam sample or two?

If Julian or any SpamCop deputies are reading, you may want to look
into that mail hosts hypothesis.

Finally, the addresses to whitelist (we have them whitelisted here) are
64.233.160.0/19.

* Thanks to Christopher Byrne for pointing this out.