I've never used gum trag as a resist but I've had great results with super sheen, are you using leather dye or acrylic paint for the letters? If you're using leather dye then would suggest doing the letters first, then sealing them with super sheen (make sure it's completely dry), then dying the rest of the item., remove excess dye smudges from the letters with a damp paper towel.
With acrylic paints I've had a good success rate with dying the entire thing, applying the finish, and then painting on top, the super sheen stops the acrylic from 'merging' with the dye or if theres any excess residue it doesnt get integrated into the paint. However I haven't used that technique over black dye, I've used it on dark blue and purple and it's turned out ok, I shouldn't think It would come out much different.
Hope this has been helpful!
http://leatherworker.net/forum/index.php?app=galleryℑ=29834
(letters and rose are purple underneath, the bell was sealed before the dye went on and then painted to see if there would be any colour difference, and there doesn't seem to be!)