Upgrading to Gutenberg is not necessarily a choice that users of WordPress will make for themselves. If you’re hosted on WordPress.com or deployed your site via a web host, you will have little to zero say about when you upgrade.
Your existing posts will be editable via Gutenberg, and will show with the entire post in a single block; be very cautious about allowing Gutenberg to migrate them to blocks, as in my testing I have found this usually corrupts the post (this may be due to how I’ve created my posts, but the conversion appears to hate any styling.
You can revert to the standard experience, by installing the Classic Editor, but any migrated posts will remain changed. The Classic Editor will remain available via a plugin, but it has already been announced that it will only be supported until 31stDecember 2021. After this the Classic Editor will no longer be supported.