Assuming the gray cast is skin oils and crud, regular household ammonia works well. Ammonia mixed with oils creates a soap and soap rinses off easily with hot water. Scrub it well with a scrub brush, rinse well and dry thoroughly. One stock I did this winter took ammonia, denatured alcohol, laquer thinner, and some judicious heating in a microwave while wrapped in toilet paper to remove the oil and dirt. Go easy on low power if you try the microwave method, I had a forend split lengthwise once, and it wasn't super hot when it split.
