How do I make kate the default text editor in gnome?

Try this little tutorial. Or, if you want a simpler way, just right click on the file and choose to open with the application you want.

"Thanks Alex this works for me on Ubuntu 16.04 LTS:

create a file ~/.local/share/applications/defaults.list with this content:

[Default Applications]
application/x-perl=kate.desktop
text/plain=kate.desktop
text/x-chdr=kate.desktop
text/x-csrc=kate.desktop
text/x-dtd=kate.desktop
text/x-java=kate.desktop
text/mathml=kate.desktop
text/x-python=kate.desktop
text/x-sql=kate.desktop

Create missing kate.desktop with a copy:

sudo cp /usr/share/applications/gedit.desktop /usr/share/applications/kate.desktop

In the copied file, replace gedit with kate i.e. with vim :%s/gedit/kate"