The short answer is - yes. While Google does make it seem like a Gmail account is required to provide a review, the requirement is simply that the user register their email account with Google beforehand.

Before writing a review, Google will ask you to connect an email. You will be asked to "Create a New Account", where you will then enter the email of your choice. Once on this page, you have the option to select "use my current email address instead."

Scroll on the knowledge panel to Reviews and you will be promoted to rate and comment on the business you have selected. Additionally, you may add any photos of your choice, which you believe may help others customers when making a decision. Once that is complete, publish your review to be visible to web.