if they're using hashed emails for tracking, we're pretty much clucked. There is an option in gmail that can prevent this. if your email is abcd@gmail.com, use abcd+sitename@gmail.com (gmail will ignore the stuff after the + ). You can also use . separators. Now, to keep track of that...
Gmail filters out '.' within usernames (e.g., john.smith@gmail.com and johnsmith@gmail.com are sent to the same account), but this gives you a more limited number of options, and is even harder to keep track of!