postinst is in gui-apt-key 0.4-2.2.
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 5 6 | #! /bin/sh
set -e
test -x /usr/bin/update-menus && /usr/bin/update-menus || /bin/true
test -x /usr/bin/update-desktop-database && update-desktop-database -q || /bin/true
|