So about a week ago at Midrealm RUM (http://midrealm.org/rum/) I managed to confuse my entire class by mentioning that the triangle piece inset into the skirt of my gown was a godet. So I sort of confusingly explained with no visuals the difference between a gore and a godet. Here is that same description, now with visuals!
Godet: A triangular shaped piece inset between two pieces of fabric or into a slit in a piece of fabric to add volume to the hem of the garment. For instance the triangular pieces sewn into a tunic or gown to widen the hem.
Gore: A typically trapezoid shaped piece sewn to other similarly shaped pieces to create a garment, either a skirt or dress. For instance the many gored gowns from the Herjolfsnes find.
Gusset: A triangle or diamond shaped piece inset between two or more seams to add room to the seam, especially in a sleeve. For instance the square pieces used under the arm on most tunics.
Now that we know what each of these words means it is easier to understand how they are not interchangeable. Gusset should be somewhat obvious and I do not believe I have ever heard anyone use gusset to describe either a gore or a godet. But it is fairly common for the word gore to be used to describe both an actual gore and a godet. While the I am aware that I am being slightly nitpicky about the use of these two words I find that identifying them by their correct names helps to decrease confusion. Part of why I prefer to make the distinction between words is that when I write out a pattern that has godets, if the other person is aware of the difference instead of making a triangle shaped piece I can write “Four godets 20” long by 18” at the base, plus seam allowance” (this is a fairly typical measurement for a knee length tunic). Actually for a tunic pattern I will typically not make paper pattern pieces unless I need a shaped armscye and just trace the pattern directly onto the fabric, but that’s another post.