Beefy, I am by no means a decent GS sculpter, but I can fill a gap. Get a few dental tools (auto parts store are good for this). Maybe some small rubber shapers from the craft store. Toothpicks work in a pinch as well.
Mix the GS together, let it set for about 10 minutes before you try to do anything with it. The first 10 minutes it is like fresh-chewed bubble gum. Sticks to everything but what you want it to stick to. Get a small cup and keep your tools wet. (olive oil will work too, but may affect adhesion of layers)(spit also works if you are of the brush licker type anyway).
Push a bit of GS into the gap, push a bit more than you need and start pushing around the extra until it looks like the rest of the area around the gap (scales, feathers, armor, etc.). You mentioned feathers (large gryphin?), that is not going to be easy. Do a very small amout at a time and make each one right, then add a bit more GS and work your way around the seam.