/usr/share/icewm/icewm-set-gnomewm is in icewm-common 1.4.3.0~pre-20180217-3.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28  | #!/bin/sh
for arg
do
    case $arg in
        -h|-?|--help|--?)
            echo "Usage: $0"
            echo "Set the Gnome windowmanager to IceWM"
            exit 0
            ;;
        -V|--version)
            echo "$0 VERSION, Copyrighted."
            exit 0
            ;;
        *)
            : echo "$0: Ignoring argument '$arg'." >&2
            ;;
    esac
done
echo Previous window manager:
gconftool-2 -g /desktop/gnome/session/required_components/windowmanager
gconftool-2 -s /desktop/gnome/session/required_components/windowmanager --type string icewm
echo New window manager:
gconftool-2 -g /desktop/gnome/session/required_components/windowmanager
 |