How did you install Fluxbox? I'm really a Debian guy but the rpm package should install a session file for Fluxbox.
The directory may be different on RedHat, but here's the general idea - create a new file called fluxbox in /etc/gdm/Sessions/, then edit it and add something like this:
# First try for a fluxbox Session file for gdm
exec /etc/X11/Xsession /usr/bin/fluxbox
Your paths may be different - type which Xsession
and which fluxbox
in a shell to get the correct locations.