Email Tracking - GMail

I am creating my own email tracking system for email marketing tracking. I have been able to determine each persons email client they are using by using the http referrer but for some reason GMAIL does not send a HTTP_REFERRER at all!

So I am trying to find another way of identifying when gmail requests a transparent image from my server. I get the following headers print_r($_SERVER);:

DOCUMENT_ROOT  =  /usr/local/apache/htdocs

GATEWAY_INTERFACE  =  CGI/1.1

HTTP_ACCEPT  =  */*

HTTP_ACCEPT_CHARSET  =  ISO-8859-1,utf-8;q=0.7,*;q=0.3

HTTP_ACCEPT_ENCODING  =  gzip,deflate,sdch

HTTP_ACCEPT_LANGUAGE  =  en-GB,en-US;q=0.8,en;q=0.6

HTTP_CONNECTION  =  keep-alive

HTTP_COOKIE  =  __utmz=156230011.1290976484.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); __utma=156230011.422791272.1290976484.1293034866.1293050468.7

HTTP_HOST  =  xx.xxx.xx.xxx

HTTP_USER_AGENT  =  Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.10 (KHTML, like Gecko) Chrome/8.0.552.237 Safari/534.10

PATH  =  /bin:/usr/bin

QUERY_STRING  =  i=MTA=

REDIRECT_STATUS  =  200

REMOTE_ADDR  =  xx.xxx.xx.xxx

REMOTE_PORT  =  61296

REQUEST_METHOD  =  GET

Is there anything of use in that list? Or is there something else I can do to actually get the http referrer, if not how are other ESPs managing to find whether gmail was used to view an email?

Btw, I appreciate it if we can hold back on whether this is ethical or not as many ESPs do this already, I just don't want to pay for their service and I want to do it internally.

Thanks all for any implementation advice.

Update

Just thought I would update this question and make it clearer in light of the bounty.

I would like to find out when a user opens my email when sent to a GMail inbox. Assume, I have the usual transparent image tracking and the user does not block images.

I would like to do this with the single request and the header details I get when the transparent image is requested.