Synaptic, kwrite,…
user@host:~$ sudo synaptic
No protocol specified** (synaptic:4508): WARNING **: Could not open X display
No protocol specified
Unable to init server: Could not connect: Connection refused(synaptic:4508): Gtk-WARNING **: cannot open display: :0
Whonix-Gateway-13.0.0.1.1
Whonix-Workstation-13.0.0.1.1
Ubuntu 16.04
KVM, Virt-manager