On Thu, Sep 21, 2017 at 11:12:19AM -0400, Renaud (Ron) OLGIATI wrote:
> Under Debian, I could su to root in a console, and launch gparted from the CLI.
>
> Now, I get:
>
> ron@ron:~/Desktop $ su
> Password:
> No protocol specified
> xmodmap: unable to open display ':0.0'

Try this:

xhost +local:
su -

Douglas.