To send HTML mail using Gmail, you need to create a Google Doc first with exactly how you want your email to look.
After that, simply copy and paste the whole page into email body.
Remember to write your texts inside a table cell as without this, the colored background/border won’t retain.
You can extend this process to make multiple columns too. Simply make multi columned table and make the border color white.
You can use this processes to send newsletters combined with mail merges.