Cannot connect to X Server when running app with sudo

You need to allow the root user access to the X server:

xhost local:root

And point the command to the right DISPLAY:

sudo DISPLAY=$DISPLAY gedit /etc/profile

Finally the problem is solved. I have just added the line:

Defaults env_keep="DISPLAY XAUTHORITY"

at the end of visudo.

To do that you have to run

sudo visudo

it will open the file then add the above line at the end.

Thanks to all, specially @Thor for kind help

Supplementing @Thor's answer:

Run xhost local:root before sudo gedit. To make this permanent, add the line

xhost local:root

to the file ~/.xinitrc ¹. Create the file if it doesn't exist.

[1] https://askubuntu.com/a/720120/452398

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