To change the default font in gedit:
Press the menu button in the top-right corner of the window and
select PreferencesFont &
Colors.
Uncheck the box next to the phrase, "Use the system fixed width
font."
Click on the current font name. gedit will open
a font-chooser window, allowing you to see available fonts and choose the
one that you prefer.
After you have chosen a new font, use the slider
under the list of fonts to set the default font size.
Click Select, and then close the dialog.