Probably your email got leaked, based on the email and the filter from google it 100% you've been spammed. If you take a look my email hmmm I have ton something like that especially local casino I like watch movie and sometimes netflix doesn't have movies that I want so I usually open illegal sites from the web and the email similar like that is sit on my spam folder.
The best way is just don't open the link if you think you never registered to that site
Email leaks might be one of the reasons to get the email. It might be leaked by sharing on a common location where the public might be able to see the email and then used to promote any site whether that is legal/illegal, right/wrong. I have no problem with that as I do not use my email anywhere which email I use to create accounts on gambling sites. As a result, there is no way to face the issue like OP.