A gore is a triangle or square of fabric inserted in the a garment to facilate movement.
Making gores for the Bottom Hem of A Tunic
The easiest way to make gores is to take any rectangular scraps of fabric left over from the body of the tunic and cut them diagonally to make two triangles. Sew these triangles into the side seam of your t-tunic with the small end at the waist and the long end at the bottom hem. It's a bit tricky to get the top in neatly by machine - either handsew it, or just do the best job you can - the St Louis tunic is a very finely tailored 14th century guys' shirt cut to a t-tunic design, and despite the rest being sewn nicely, there are apparently still wrinkles at the top of the gores.