As an anecdotal example, I have an email address that's been strewn about the internet for almost 2 decades. It's currently hosted on Google Apps, but it has a non-Google domain. I get a spam message in my inbox maybe once every couple of months.
I also have a gmail address. I never use the thing. But it gets inundated with spam, and every month or so when I go look at it I have to clean lots of junk out of the inbox.
Since they're both hosted by Google, I'm forced to conclude that the gmail one gets many orders of magnitude more spam merely by virtue of ending in @gmail.com.