The risk of loosing a domain, especially if used only for email, is lower than losing a google account. Using a gmail.com means that google owns both your emails and your email address and can do whatever they want with it.
Even if it's just your google account being locked for some random reason, good luck getting out of the situation and/or getting in touch with a human there.
If you can't access your gmail.com address anymore then you become locked out of so many other things.