Step 1 You need to code HTML with embedded* CSS styles and Images are hosted* on your server that accessible* publicly.
Step 2 Open your .html file (ready newsletter) in a text editor. You can use free Notepad++ for that.
Step 3 Compose a new draft message in GMail and open developer tools: Type here anything you want (e.g.: CODE HERE). Then Highlight the text, right-click it, and press “Inspect.” This will open the developer tools that allow editing the code right in the browser. Highlight the content inside and select the “Edit as HTML” option. Find the “CODE HERE”, Highlight the text and Replace it with the code from your .html file. Confirm the changes. That is all. Now you should have your newsletter inside the body of your email draft.
With GMAIL: There is a limited amount of emails that can be sent from your account. You are allowed to send from 500 (unpaid email account) to 2000 emails per day. As the best practices for conducting successful email marketing campaigns, recommendation is that you should not use your Gmail account for sending out email newsletters since this will reduce credibility and trustworthiness. Also, using personal account for commercial purposes is not a good practice.