I agree with Patches1900-I use matching thread, not white or neutral for all, and take tiny whip stitches, making sure it is pulled tight enough not to pucker. There is nothing wrong with cotton thread, silk may disappear a bit more, but it is also very expensive and harder to work with. I am also making a GFG, and my stitches hardly show, using matching threads and taking tiny whip stitches (only 2 or 3 threads).