Abedul

The main difference between non-NT in GMP and NT maps I'm aware of is RGN5 block is only present in NT-format maps.
RGN5 consists of a number of Huffman tables used to compress polygons' and polylines' coordinates. The structure of these tables as well as the decoding algorithm
are yet to be explored.

To check if jnx2img really produced a NT-map, you can open LOG file in TEMP folder of the converter, and search for 'huffman' word.
If you see something like this:
Bytes used by rgn huffman area geometry: 277
then the IMG is an NT map.
And if the number of bytes is zero, then non-NT map was made.