This G'Mic bash script allows for turning a folder of images into a montage. Can be used with Thunar Custom Actions.

gmicmontagefolderimages.sh

#!/bin/bash

find -type d -exec sh -c '
  cd "$0"
  set -- *.jpg
  if [ -e "$1" ]; then
    gmic "$@" -fx_montage 4,\""V(H(0,1),H(2,V(3,4)))"\",1,1.0,0,5,0,0,0,255,0,0,0,0,0 -o 0000."$(date)".jpg
  fi
' {} \;