Maybe!!
I've done email marketing with tracking where I designed the lot (the tracking system, the email, the databases etc). The idea is you include an image, when you download the image it sends details which is then executed on the server.
However, you must realise that, unless the technology has moved so quickly I'm behind, you have to download the pixel. If you don't download the pixel (the pixel means image) then it won't work. And email clients like Outlook often automatically prevent images from downloading!
So, is it secure? That depends on your set up and security. If you have an email client which allows me to run javascript or even execute macro's etc, then I would say no. And I could exploit it.
The other issue is you may have is, the third party (in this case Google), may be able to track you for your own needs (and I'm not suggesting for the power of evil). For example, tracking programs often include the users email so you know who opened the email. Emails are unique so maybe Google can also store this information gathered from your mail shot. Then, if the client has Gmail, the next time they log on to Gmail, they see adverts more tailored for their needs. This could even be showing your competitors.
Now, and this paragraph is opinion based, since you are doing this via Google, and I would say they are a fairly big company that the likelyhood is they are very security conscious. Having a vulnerability where data was leaked in the manner you describe would greatly damage their reputation; not something they would want to do. However, doing the same thing via Dodgy Dealing Enterprises may result in something less than ethical!