> what should IANA do when SSH registers a new GSS KEX
There's no such thing. GSSAPI mechanisms are identified by OIDs, which are assigned by the owner of the parent arc. There is no central registry, and certainly not one run by IANA.
SSH can be used with any GSSAPI mechanism that includes the features it needs. No registration is required or possible.
The same problem affects other algorithm and method identifiers, too. All of these namespaces include provision for privately-assigned names, and it's common for algorithms with such names to become widely deployed and even considered best practice without any sort of registration.
-- Jeff