In the classic editor of WordPress, you can easily copy the entire content by right-clicking the post, select all and followed by copying it.
You can’t do this with the Gutenberg Editor. This is because when you write a post using the Gutenberg Editor, it will split into a few different blocks. So, when you try to right-click and select all the post to copy, it will only select all the contents that belong to the block which you have right-clicked. Here’s an example of what I mean:
You can, however, do the old method of copying which is selecting all the contents manually and followed by right-clicking on it and copy.
But I advise you not to do this because you might miss selecting some of the contents.
What I advise you to do when it comes to copying all the contents in Gutenberg Editor is by making use of the hidden copy feature that’s available in the Gutenberg Editor. This hidden copy feature can be found by clicking on the three dots button located at the top right-hand corner of the editor.
By clicking on it, a dropdown menu will appear with plenty of options to help you work with the Gutenberg Editor better such as fullscreen mode, spotlight mode and etc. One of the options is Copy All Content.
By clicking on it, the entire contents in the Gutenberg Editor will be copied without anything being missed.