GCode Sequencer is a little project I wrote in a couple of days. It takes pre-sliced gcode for individual models, does a basic parse of the gcode to work out the axis alligned bouding box and places each of the models on the build plate as densely as it can.
The program is designed for Ender 3 style FDM printers with a single x axis alligned gantry and a print head assembly that sticks out in the -y direction. It will take into account the printer clearances in deeming if a location is valid such that the printer will not crash into previously printed parts.
For more information on this project please see the github repository page where there is more detailed information on the specifics of the program.
This is a printed example of what the program can create. They are parts for the spyglass from A Series of Unfortunate Events that I have replicated and have been selling. (note the build plate has been rotated such that -x is on the left)