Paste without formatting while composing
Ctrl+Shift+V pastes the text with Gmail's default font and size, without the bold, colors or backgrounds carried over from the original website or document.
Remove formatting from text you already pasted
Select the text and press Ctrl+\ (⌘ Cmd+\ on Mac). You can also open the formatting options (the “A” icon in the bottom toolbar) and click Remove formatting.
Send the email in plain text
In the compose window, open the ⋮ More options menu and turn on Plain text mode. The email will be sent without any formatting, not even bold text or links with anchor text.
If the shortcut doesn't work in your app, paste the text in the box above, click Copy plain text and paste it wherever you want: it will arrive without any formatting.