When you use command line to pull, push or clone code to your server, if you see this error:

(gnome-ssh-askpass:13543): Gtk-WARNING **: cannot open display:

To solve this, all you need to do is run the following command in the terminal:
$ unset SSH_ASKPASS

To prevent it in future, you can add the above line in your .bashrc or .bash_profile.

Wish you trouble-free working!