First Insertion Method
Or, Two Seam Insertion Method
This method starts at the apex of the slash, and sews each side of the gore separately. This is the method I prefer to use with heavier materials, like coutil.
Cut your gore slash.
With right sides together, match up the dot on your gore with the apex of the slash. Insert your sewing machine needle at the apex. Stitch forward and backward a couple times to secure, being very careful not to back up beyond the apex.
Stitch the seam, keeping the gore at seam allowance and following the seam line for the slash. Back stitch to finish at the edge of the fabric.
This is what your gore should look like after finishing the first seam. The stitch should follow the seam allowance from the dot to the top edge of the gore. The body of the gore should be right-sides together, with the bulk of it away from the slash.
Turn the gore so the seam is open, pulling the seam allowance of the gore at the bottom of the slash from the right side to the wrong side of the garment.
Fold over the panel of fabric so you are looking at the second seam, right sides together, with the gore underneath.
Start at the apex of the slash like you did for the first side, and stitch to the edge of the gore. Be sure to follow the stitch line and keep the gore at seam allowance.
Open it out, and your gore should look something like this.