I need/want to ssh to the gumstix. I used to be able to do it, but
after configuring for the mmc rootfs, I seem to have deleted whatever
it takes. I've looked through the buildroot/kernel/busybox config
options but can't find reference to ssh support.

The symptoms of trying to do ssh to gumstix is that I get a "connection
refused" message. What am I missing?

The gumstix uses the ssh server 'dropbear.' Just enable that package and you should be fine.