Monokai for Gedit is a theme for GtkSourceView based on Monokai Extend for SublimeText.

Monokai in Gedit

You can download it here: https://gist.github.com/LeoIannacone/71028cc3bce04567d77e

Then move the monokai-extend.xml file into your ~/.local/share/gtksourceview-3.0/styles/ and enable it by selecting “Monokai Extended“ in Gedit → Preferences → Font & Colors