Trying to save any change ends in "failed to save gist"
Console isn't helpful at all.
Hey there.
Before anything, please note that this is just some old pet project. I'm not really developing or mantaining it.
As for the issue you mentioned, it seems you didn't "log in". The login is just some textbox where you fill in a github token with gist permissions. When you click login there's a popup with a convenient link to the personal access tokens github page. I just did a quick test and to my surprise it's still working.
Weird, I never saw any log-in prompt ;)
In case this project is no longer maintained, GitHub now has an "archive" option (which just displays a big banner at the top, it's not like it deletes or hides the repo)