Go to your Google Account
Sign in if prompted
Click Data & privacy
Scroll to Data from apps and services you use
Click Delete a Google service
Sign in again if prompted
Find Gmail and click the delete icon
Enter an email address that is not a Gmail address for future Google sign-in
Verify the email address
Open the verification email and confirm
Follow the on-screen prompts to delete Gmail
Save any important emails before deleting
If needed, download your data first using Google Takeout