A gore is a triangle of fabric inserted in the "skirt" of guys' and girls' t-tunics to allow more space to move the legs. Girls tend to have more gores than guys since their tunics are longer (unless they dress as a boy-which is fine), but guys generally also need a couple of gores so that they can easily run, dance and fight in their 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.