Hey, if you're still looking for this. I tried the lua one too and was also having trouble, so I wrote my own in python using the key from Zantherz's program.
It uses python 2.7. Pass it, as an argument, your encoded(PC) save or decoded(switch) save and it will output either Decoded.save or Encoded.save.
So just run either:
python C:\Path\to\GungeonSaveConverter.py C:\Path\to\SlotA.save
or
py -2.7 C:\Path\to\GungeonSaveConverter.py C:\Path\to\SlotA.save
depending on whether or not you've got python 3 installed. Obviously replace "C:\Path\to\" with the actual path to those files.
Then put the Decoded.save into a dumped save from your switch, rename it to SlotA.save (or whichever slot) and reinject it on the switch.