Open Gmail in your browser
Open the email you want to download
Click the three-dot menu in the email header
Select Download message
Save the .eml file to your computer
To download multiple emails, select them in Gmail
Click the three-dot menu in the toolbar
Choose Download messages if available, or use Google Takeout for bulk export
To download all Gmail data, go to Google Takeout
Select Mail and create the export
Download the exported archive when it is ready