assigning a static IP in Debian is easy..
user@it-etch:~$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
iface lo inet loopback
# The primary network interface
# allow-hotplug eth0
# iface eth0 inet dhcp
iface eth0 inet static
user@it-etch:~$ cat /etc/resolv.conf
I don't use the GUI to manage static IP's, it's fairly straight forward in the configuration file.
You will probably need to toggle the interface after making the changes..