Has it? If Google's proxy is caching images, then "email open" tracking might have broken entirely. All the sender would see is that their email has been opened once by the proxy -- for all gmail addresses put together.
Or, if they snip the GET variable for whatever reason (I don't see them doing this):
http://example.com/gmail/{yourusername}/trackingimage.php
Or even:
http://example.com/{emailidfromadatabase}/trackingimage.php
This tactic is already in use by most mass email companies.
A solution would be 1-pixel high tracking lines - a 1 x 128 pixel wide image that encoded 0 and 1 as two RGB colors adjacent to the mail's background color in the visual spectrum so the difference isn't noticeable would encode a sha-1 hash placed in the url.
http://example.com/tracking-line/{hash}.pngThat would essentially render open statistics meaningless and would let Google cripple another industry after the promotions tab and 'not provided.'
I really hope they don't because it's such valuable information when creating email copy...
first one = Google
second one = user