Remove formatting in rich text with Gutenbergs Block editor

I’m afraid there is no such function in there yet.

You can find this bug: https://github.com/WordPress/gutenberg/issues/8869 and it’s still open as enhancement.

On the other hand, you can paste using Cmd+Shift+V (Ctrl+Shift+V on Win) and it should paste your text ignoring most of formatting, I guess.