Okay, here's the problem. GRT is based on internal volume, not weight. It is a number decided upon by the Insurer and the ship's Owner. It decides what the Insurer will pay to the Owner if the ship sinks, and it decides what the sub captain who sank the ship will be officialy credited with.
The number listed for the ship in the OP is Deadweight tonnage, which is how much cargo the ship is actually expected to carry. There can be a huge difference.
Here the modern Gross Tonnage is smaller - not quite 12,000.
http://www.shipspotting.com/gallery/...php?lid=401151
Given the ship's length - 150m - I would have guessed the GRT to be around 7000, the same as an Empire or Liberty ship; but the design is more modern and obviously built to maximize cargo space in ways not fully understood sixty years ago.
Second look: Here is the Wiki on Liberty Ships. Note that
Nogat is both longer (150m vs 135m) and wider (25m vs 17m) with roughly the same draft (8.8m vs 8.5m). And note that the Liberty shows a displacement tonnage of over 14,000 tons, roughly twice the GRT which was about 7200 tons.
http://en.wikipedia.org/wiki/Liberty_ship