OK, I understand. I just ran a test with my setup, and it’s quite a mess, so @emyoulation is quite right with this comment:
Anyway, to navigate this fine mess, your best bet is to rename the original .gpkg to .gz and open that inside Nemo. It will then automatically open the archive manager, called Archiefbeheer here (I’m in The Netherlands). And when you use that, you’ll see another .gpkg file inside, which you can extract. You could also use gunzip for that, but using the GUI is often easier to see what you’re doing.
Now the file command reveals that this .gpkg file is actually a TAR file, so you need to do another rename to .tar, and open that with the archive manager again. And this will finally reveal a list of all the files in this archive (TAR means Tape ARchive!), from which you only need to extract the data.gramps file. You will also recognize your media, but I assume that you have those already. If not, extract all files.
The file command revealed that this data.gramps is again a compressed file, so the story repeats itself with a rename to data.gz and running the archive manager, or gunzip, to reveal a genuine uncompressed data.gramps, which should contain XML text, and look like what @DaveSch wrote.
There is a chance that you won’t get this far, because the archive itself has been corrupted.
And this time, life on Windows would be a lot easier, because on that you can let 7-Zip do all the work.