I made a software for combining layers from different G-code files into single file. Currently it's only available for files generated by Ultimaker Cura, but I will add support for other slicers in near future. You can use it to make retraction towers or other tests in only one print. Feel free to share your thoughts and issues so I can fix them :)

G-code Combiner is available for download on the Github:

Links to VirusTotal scans are in the readme file :)