I don't know if I'm being dense, but I can't see how to view a post from the Gutenberg editor without either editing it first and updating, or manually changing the URL.
They don't hate us that much do they?
The editor is in full-screen mode by default and hides the top toolbar that displays the view page button.
You can disable the full-screen mode by CMD (or control) +Option+Shift+F
or ⇧⌥⌘F
or by opening the 3 dot button in the upper-right hand corner of your screen.