In reply to Gary.

But, as previously asked, what will happen if I don’t install Classic Editor plugin and simply click on “dismiss”?

If you just dismiss the notification, it'll be hidden. If you ever need to bring it back at any point, you can find it in the "Screen Options" dropdown at the top right of your Dashboard.

Gutenberg itself will be released in WordPress 5.0, so if you don't install the Classic Editor plugin, it will become your default editor when you click the upgrade button. (WordPress 5.0 will not be an auto update release.) You can still choose to install the Classic Editor plugin after upgrading to WordPress 5.0, should you find that something isn't quite working right.