I think the documentation you should be following is the Tails backup instructions found here. I find that it is more helpful and up to date. I believe these documents referencing gksu are out of date, I think the Tails docs are out of date. We have opened a ticket to update our docs on our GitHub, you can follow the ticket there.
Thank you @bmeson for the answer and the GitHub ticket.
When there is something you are not that familiar with and there is a documentaion you tend to strictly follow it, step by step. And when something goes wrong you think “What did I do wrong now”.
I totaly agree with your thought on just referencing to the tails documentation regarding the backup steps, why rewrite something that already exists. Something that might change at anytime, out of your control.