I see what you're trying to accomplish. You're trying to connect directly to the hook tube. Sorry, but the traveler series isn't designed that way when using a 3rd party head.
The traveler has been designed to be used as a kit with their head, which unfortunately, no one, including me, seems to like.
That being said, I've figured out where the "missing disk" is. It's actually there, attached to the head with three small bolts.
The description of the tripod states for the Ground Level Set, "This new feature allows you to remove the centre column to save weight and get the camera closer to the ground. Uses existing components: the upper disc with reversible 1/4'' - 3/8'' bolt and the column hook assembly: Remove column and fix the disk directly to the upper casting using the column hook. No tools needed."
When you remove the kit head, on its bottom is a black disk attached to the rest of the head with the 3 small bolts mentioned above. Sticking out of the disk is the stud which attaches the kit head to the center column. What they are talking about is remove the head from the center column, then unscrew the hook assembly from the column, then remove the column. Finally place the head with the disk and the stud sticking out on top of the hole where the center column was. Screw it into the hook assembly. Once tight, the kit head is very solid and won't wobble at all.
That's using the Ground Level Set. While I haven't taken the kit head apart in a long time, I don't remember the black disk on the bottom of the head being useful or usable for third party head attachment.
At this time, the only thing I can suggest is using the GS1510KB short column which Gitzo says on their website will fit the Traveler 2 leg set.
I'm wondering if the GS2320D upper disk kit or the GS1320KT traveler 1 ground level kit might work for you. I have no idea if either will work myself, and haven't tried to accomplish what you're trying to do. Thus far, it's been outside of my needs.
While I often find Manfrotto Distribution Inc. the Gitzo US and UK representative tough to contact, I think you need to contact them and ask if either of the two solutions above will work, unless you have a way to try them, and if they don't work, return the parts for refund.
Let us know what you determine please, as it might help another Nikonian in the future.