This G'Mic script will go through a folder of many images and output multiple montages in the working folder. The script can be modified to customize the number of images to turn into individual montages by changing the "4" in "(expr $c % 4)" to whatever number desired. Can be used with Thunar Custom Actions.

gmicmontagepics.sh

#!/bin/bash

c=0
for i in *.jpg; do
  c=$(($c+1))
  f="$f $i"
  test "$(expr $c % 4)" != 0 && continue
  gmic $f -fx_montage 4,\""V(H(0,1),H(2,V(3,4)))"\",1,1.0,0,5,0,0,0,255,0,0,0,0,0 -o output-$c.jpg
  f=""
done