One simple reason for the draft coming through the electric boxes is poor air barrier installation.
The solution may not be as simple. I’m not all to familiar with gaskets, however, an expandible foam insulation may be your easiest solution.
You would have to remove the cover plate, then insert the tip of the foam container..(looks much like a hair spray bottle, upside down)..through an opening
in the outlet box and spray short blast of expandible foam behind the box. Too much isn’t a good thing necessarily, however you’ll need enough to seal off the
space behind the box.
You can ask for expandible foam insulation at your nearest supply yard…