This is the most common manual method. It involves saving the email as a PDF first and then converting it to text.
Click (three vertical dots) and select Download message . Download gmail txt
2 Ways to Export Gmail to Text File with Attachments in Bulk This is the most common manual method
While not a .txt file, Gmail's native download feature saves the email in format, which is a raw text-based format readable by most text editors. Open the email. Download gmail txt