Let’s draft together the pattern of this long dress.
Start from a basic block with darts.
On both front and back reduce the skirt from the sides to make it tighter, draw the round cut outs at the sides and the new neckline.
On front only, widen the shoulder dart by 1cm at the neckline.
On back only, discard the dart from the back and side.
On both draw the slash lines for the gathering.
Cut through all the lines and on the slash ones cut from the center towards the side.
Open few cm in each cut.
Redraw both back and front filling the gaps and marking the gathering positions with notches.