Turn on syntax highlighting
gedit provides syntax highlighting for a wide range of markup, programming, and scientific languages. If gedit recognizes the syntax being used when you open a file, it will automatically highlight the text.
If your syntax or language is not highlighted upon startup, you can select the appropriate syntax or language by clicking View ▸ Highlight Mode, and then choosing the desired syntax. Alternately, you can select the syntax name from a list at the bottom of the gedit window.