4: in condor_config set SEC_DEFAULT_AUTHENTICATION_METHODS = GSI
and that should do it on the HTCondor daemon side of things.
+Optionally, edit =/path/to/gsi_keys/mapfiles/grid-mapfile= and replace =USERNAME_HERE= with your username. This is not necessary to get the daemons to start up, but needed for condor_submit.
+
On the client side, for simple testing without the need for grid-proxy-init and friends, you could just set the following environment variable for the tools to use:
setenv X509_USER_PROXY /path/to/gsi_keys/hostproxycert