This might have been the plan, but it doesn't happen in my experience. I've tested it multiple times and Gmail loads those images when you open the email and it's possible to track when users open their email. It also appears that Gmail loads those tracking pixels every time you open that email, doesn't do caching, so it's possible to track how many times the recipient opens the email.

The redeeming quality is that by proxying those images, your IP and User-Agent are no longer leaked. But that's it.