Likely an unexpected encoding:
Thefile command can tell you the encoding used:
ISO-8859-15.txt: ISO-8859 text
With this information you can use iconv to convert to another encoding (usually UTF-8):
iconv -f ISO-8859-15 -t UTF-8 -o UTF-8.txt ISO-8859-15.txt
Probably this has to do with a different character encoding (see https://en.wikipedia.org/wiki/Character_encoding) being used when the file was created, and the gedit editor is not able to guess that.
One way to solve it could be to try opening the file using some other program, like emacs or libreoffice -- from there you can then hopefully save the file ...
A more detailed answer from:
How to debug c programs by gedit?
Use gedit external terminal plug-in
You can use gedit with terminal plugin. The steps are fairly straight-forward:
Enable "Universe" repository
Activate "Embedded Terminal"
Use Ctrl+F9 to open terminal
Other gedit plug-ins
Step 1. Enable "Universe" ...