If I open a file from nemo it opens in gedit as expected, and then if I open another file, it opens in a new tab in that same gedit instance.
However, if I first open gedit from the applications menu, and file>open a file, it opens...but then if I open a file from nemo it opens a new instance of gedit instead of putting it in a new tab in the original instance.
Am I missing an option here or is this a bug? Not sure if this is significant (or if it's exactly the issue) but in the applications menu, it is listed purely as gedit, whereas in nemo right-click/open with it is "Text editor" instead. (Although if you go to a file's properties, the open with tab there for selecting a default application lists it as gedit instead of text editor)