Here is how you can leave a review on google, without a gmail account.
Step 1: Go into Google Maps and search the business you'd like to leave the review for.
Step 2: Connect the email of your preference.
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."
Step 3: Write your review & Add Relevant Photos.
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.