I can't find the option to enable line numbering. It'd be very helpful!
I had this problem on Ubuntu and just to spare another unobservant person the time I spent digging around for it I thought I'd add an answer here as well.
If you go into preferences you can see that the option to show line numbers is grayed out but if you hover over it it says to install python-gtksourceview2.
So on ubuntu:
sudo apt-get install python-gtksourceview2
And you should be good to go.