So here it is:
To install just extract archive to ~/.local/share/gtksourceview-3.0/language-specs directory.
Adapted for Context Free 3 and gedit 3.
If you have any remarks and suggestions, I'll be glad to hear them.
As a bonus, here are the simple script to make contextfreeing in gedit easier:
png_file="$GEDIT_CURRENT_DOCUMENT_DIR/`basename "$GEDIT_CURRENT_DOCUMENT_NAME" .cfdg`.png"
cfdg "$cfdg_file" "$png_file" -w $size -h $size && $viewer "$png_file"
To use it, choose Tools > Manage External Tools in the menu, click little "+" button and copy-paste this script in the field to the right. Then click "Shortcut Key" field, and press some key combination, for example CTRL+R. When you press it again after closing configuration window, your currently opened file will be rendered by cfdg and result presented to you.