Soak the packet of Knox gelatin in ¼ cup cold water and set aside.
Separate the 3 eggs, and place place the whites in the refrigerator to keep cold. Beat egg yoks slightly, and add the sugar, plain cooked pumpkin, milk, salt, and pumpkin pie spice.
Cook and stir these ingredients over - NOT IN! - boiling water in a double boiler pot until thick, about 10 minutes. It will stick to the spoon, sort of like making pudding. Stir in the soaked gelatin until dissolved.
Chill until mixture begins to set. You will need to keep checking as this takes maybe an hour or two. Dont' let it get too set, because then it is difficult to fold in the egg whites.
Once the mixture is set enough, then whip the egg whites and salt until they are stiff, but not dry. Fold* into the pumpkin mixture, fill the pie shell, then refrigerate.
Before serving, spread a thick layer of Cool Whip topping over the pie.
*To carefully fold in the whipped egg whites that so that they do no deflate, put the bowl and beaters into the fridge, too,before you begin all of this as they will rise higher if the bowl is cold and very dry. Put small amounts of the beaten egg whites into the pumpkin mixture, a little at a time. Do not stir!!!! Just fold from under to over gently, going all around the bowl. There may be some small bits of egg white that remain. Don't worry, that is normal. You want to avoid knocking all of the air out of everything!