postinst is in gconf2-common 3.2.6-3ubuntu6.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 | #! /bin/sh
set -e
ucf /usr/share/gconf/default.path /etc/gconf/2/path
|