How to add fluxbox to the gdm menu
Fluxbox is a Slackware package and since Slackware doesn't come with GDM, the package has no provision for a GDM menu entry. You can add one yourself by creating a /usr/share/xsessions/fluxbox.desktop file with the following contents:
[Desktop Entry] Encoding=UTF-8 Type=XSession Name=Fluxbox Exec=dbus-launch /etc/X11/xinit/xinitrc.fluxbox Icon=fluxbox
Next time you will get to the GDM screen, fluxbox will be available as an option.