I am trying to install an astronomical software package GILDAS: https://www.iram.fr/IRAMFR/GILDAS/
Following the instructions on the Download page of that web, I installed it with "sudo port install". When I tried to launch it after installation with the command "greg", I go the error message:

(<unknown>:29409): Gtk-WARNING **: cannot open display:

I used sudo to install the software but are trying to use it from my normal user account. How should I do in this case? How can I diagnose the problem?

I am using MAC OS X El Capitan. Someone reminded me that the software GILDA makes use of the X11 and thus the problem could be related to my installation of X11/Xquartz. But how to find out what is wrong?

Thanks!

You have to start X11.app then run your software from the xterm opened in this app.

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