Gnome already has a "Disk is about to become full" notification. By default, the warning is at 0.05% disk space left. However, limited users get denied further writes once the disk is 5% free (root can go right up until the very last byte on the disk).
It should probably be configured to something like 6.5% free space before the notification comes up. This value is changable: Go into gconf-editor and go to /desktop/gnome/volume_manager and find the "percent_threshold" key.