Sorry for the delay, was busy.
I changed the save file's permission bits from
644 (-rw-r--r--) to
664 (-rw-rw-r--)
(so group members who aren't "me" can also write), and GG was able to save! So that fixes the issue for me personally. But please create the save file with slightly looser permissions - I don't know what that looks like in Unity or whatever frameworks you might use.
A potential workaround - I forget the details of file permissions, but I remember vaguely that sometimes even when you're not allowed to write to a file, you're allowed to read it, delete it, and write out a new file with the same name. I wouldn't recommend this as a first option.