I am puzzled by the behaviour of gedit. I have three different machines and while in two of them I have the desired results (after opening a document with gedit the next ones are opened in tabs), in another machine I don't. I have tried the suggestions in:
Configure Gedit to always open documents in new tab
Why does gedit keep randomly opening new instances when opening files from nautilus?
gedit opens different window though it should not
I tried to find inspiration from other questions like:
gedit open second file onwards in a new window. Ubuntu 19.10
Of course I compared what looks like relevant files (/usr/share/applications/gedit.desktop --and-- /usr/share/applications/org.gnome.gedit.desktop) from my working machines to the files in the target machine, did many trials...all of them ended up in failure.
It is like gedit will either "does not matter what" open in a new tab or "does not matter what" in a new window...
I run Ubuntu 20.04.5 LTS