I forgot to add the salt in my gingerbread dough, and it's already in the fridge. Should I re do the dough? Is there a way to add it in?
Salt is primarily a flavor enhancer in cookies. It's unlikely that omitting it will affect the recipe greatly. I know people who regularly omit salt with no problems.
Bake the cookies! Don't waste the ingredients. You probably won't notice a huge change in flavor.
As a note, you're unlikely to get an even incorporation adding it at this point, so don't try that. You'll either end up with tough dough from overworking it or salty pockets if you don't.
I would suggest sprinkling salt on top of the dough just before baking.
Salt is a flavor enhancer, as Catija mentions, but for some people, and for some recipes, it may make a big difference to leave it out. And it will be difficult to incorporate the salt cleanly, at this point. But sprinkling it on before baking should give that enhancement back with a fairly even distribution.
You may wish to use somewhat less salt, as it will be more noticeable on the surface of the food. Less salt and a fine grind of salt with an even dusting technique should make the addition much less noticeable.
Alternatively, you can use the full amount of salt, perhaps even larger flakes or specialty finishing salts, which will leave the addition more noticeable but may create a very nice effect that seems to be deliberate.
If the amount of salt seems less after baking, you might use a salt wash (just a bit of water or milk to dissolve) since dry grains will be less likely to stick to the already baked surface.