How to change the default text editor of GNOME

The default file associations are set in either /usr/share/applications/defaults.list (global) or one of ~/.local/share/applications/mimeapps.list or ~/.local/share/applications/defaults.list.

For example, this is what I have on my system:

$ grep -e '\[\|plain' .local/share/applications/mimeapps.list 
[Default Applications]
text/plain=pluma.desktop;
[Added Associations]
text/plain=emacs.desktop;pluma.desktop;

Changing the [Added Associations] line to

text/plain=sublime_text.desktop;emacs.desktop;pluma.desktop;

made sublime my default editor. Make sure that sublime_text correctly launches sublime if you run it from a terminal and that you have a .desktop file in /usr/share/applications/:

$ cat /usr/share/applications/sublime_text.desktop
[Desktop Entry]
Name=Sublime
GenericName=Text Editor
Exec=sublime_text %U
Terminal=false
Type=Application
StartupNotify=true
MimeType=text/plain;
Icon=/usr/share/icons/mate/48x48/apps/sublime_text.png
Categories=GTK;Utility;TextEditor;

You can do so by modifying the Xorg.conf by adding the line

Section "Monitor"
     Identifier      "Monitor1"
     Option  "Rotate"        "left"
EndSection

or after login use the xrandr command as:

xrandr --output DVI-2 --rotate left

Replace DVI-2 and left as per your requirement

xrandr solution will be effective till session exist. On reboot, the changes will be lost whereas xorg.conf setting will be reboot-persistant