There are two possibilities:
1. you use nano instead of gedit; the command should be found:
nano /etc/default/grub
It's a little command line text editor, very simple to use. Just press ctrl
and x to exit; you will be asked whether to save or not.
answer with yes
2. you figure out why gedit doesn't work.
Does /usr/bin/gedit exist?
are you able to edit with
sudo /usr/bin/gedit
and then : file > open "/etc/default/grub"
?
If not, just do a
sudo apt-get install gedit
to make sure it is installed.
You were searching the folder manually probably with nautilus and without
root permissions.
You can try the same thing, but open up nautilus with
sudo nautilus
