Perhaps I have figured it out.
I assume GB built the boats for block ICE and a drain is std on ICE boxes.
Way back boats were delivered almost as empty buckets ,
The owner selected the reefer, auto pilot , air cond , central heat, radio and nav gear and frequently the owners yard ,or the US boat dealer ,,, not the boat builder would do the installations.
If the Grunert was later installed I can see where the yard guy would think he needed to maintain an overboard drain that already exists.
In reality it is not needed.
Plug the spot where the sea water coolant exhaust gets fed from this drain hose.
Let the box hose drain into the bilge .
There should be an external (to the box) loop in the drain line to work as a trap, and not allow cold dense air escape the box.
When you periodically empty the box to scrub it out , the cleaning water will still drain as usual.
Does this solve the hassle?