Under some circumstances, some modes might be missing. For instance,
if the monitor does not report correct EDID information. Or if the
output didn’t have a CRTC available at startup because another output
was using it and you disabled it in the meantime.
If a mode exist, you may add it to one output with:
$ xrandr --addmode VGA1 800x600
If the mode does not exist, you may first create it by passing a modeline:
$ xrandr --newmode <ModeLine>
You may create a modeline using the gtf or cvt tools (shipped in
the xserver-xorg-core package).