I think it's not possible anymore.
If I remember right, Gmail had that option, but it went away a long time ago.
It MIGHT be possible to see a plain text version if you select "Show Original" (it's in the menu after you click on the 3 vertical dots in the upper right corner of the message). Scroll down past all the email headers, and hunt around in there. There might be a section with plain text, and in my view at least, Show Original uses a monospaced font. But many email messages these days don't have a section with plain text. Most of it is HTML or otherwise encoded. And even if there is a section with plain text, it might not be the whole message.
I guess the other option is to copy-and-paste into your favorite word processor or text editing program (Wordpad, Notepad, Notepad++, etc.).
Andy