The gsettings command to make caps lock act as a control key in Gnome is this:
If you are worried about losing any other settings you might have in xkb-options, then do this:
This is an explanation I got just now (from someone who should know about this stuff):
"GNOME 3.0 shipped with input methods separately from xkb, now the two have been brought together to have one dialog. I suspect xkb options will come back, but that'll take another iteration"
Ps, for more info on gnome input sources, see these blog posts by mclasen.
Pps, I've been an anti-caps lock campaigner for a while now :)