I'm not converting an image as you have mentioned. So I think we need to look at this again.
Thanks for getting back. The sample movie I sent showed exactly how to create a negative GDSII file from your Gerber, so I'm not sure I understand your confusion.
It would be nice if GDSII behaved just like Gerber (and handled negative images), but it does not... there are certain concessions that you have to make if you want to convert to GDSII. GDS-II does not support "negative images", thus to make your design negative (for a GDSII conversion); you must draw an outer polygon around whatever image you wish to convert and reverse the polarity of the inner objects.
Note: When you load a "Negative" Gerber file and then look at it, can you tell me where is the border where the black area (i.e. negative fill) begins?? There is no border in gerber, the negative fill stretches out to infinity.. GDSII cannot support anything to infinity, it must have a defined boundary which can be used as the negative fill, and then you reverse the polarity of the inner objects --- resulting in a negative design that can be outputted to GDSII.