This helps in that you don't connect directly to the image so it doesn't leak your IP and other info that would be available from the connection.

Marketing emails still send unique URLs for each recipient so they can associate your email address with opening any images and links. Google's proxy doesn't remedy this.

Won't the image proxy make it look like all emails to Gmail addresses are always opened, essentially making the tracking useless?

As far as I know, Gmail doesn't load the images until the first time the user opens the message, so it unfortunately doesn't make the tracking as useless as you could hope.