I don't have GC, but I would imagine you can export a 16-bit TGA, open it up in Photoshop, make sure it is set to 16-bit grayscale, then save it as a 16-bit TIFF, then follow the directions above. See if that doesn't work for you.

I think GC has some 'other' way of using 16-bit TGAs in Vue, using an import bitmap tile in the filter editor. Anyway, try the above and see if it works for you.

The other option is to export as RAW, and import into Pshop, and export as 16-bit Grayscale TIFF.