Hmm, there are a few options that might help. It's very common for people to use their generic Gmail/yahoo emails when signing up for blogs/newsletters for instance. What I would suggest is to update the email address form field copy so it says "business email" or "work email" instead of just "email address" to encourage them to use a non-Gmail address. You can also put the helper text as "please include your work/business email" instead if that works.
Alternatively, add in a "company name" form field. At the very least, even if their email is a Gmail address, you will still know the company a new lead is from.