As far as I'm aware, 'gut-loading' as a special procedure isn't really going to be that helpful. Just have a well-fed feeder; maybe use natural food items if that's what floats your boat. Gut-loading gives the impression that you're doing something special, but for insectivores, with few exceptions, it just means feeding the crickets decent whole foods (carrots, potatoes, greens) that you'd feed if you wanted to have healthy crickets. Maybe dusting them prior to feeding depending on what's eating them. For mice I imagine the equivalent would just be to feed them well on as close to a natural diet as you can reasonably do.
I don't raise my own mice though, so there may actually be a way to feed them that's healthier for the snakes and I've just never heard of it.