Slice of New York (theres one on both sides of you in Seal Beach and Huntington Beach) IMO its as good as you'll get in the area as far as NY style goes.
If you want strictly "in" sunset, Stoneys is pretty good and sells by the slice.
A Slice of New York Pizza on Algonquin (in the shopping center) has gluten-free pizza that is pretty decent. You can even get it by the slice which never happens. Their chicken wings are also gluten free. And they even have gluten-free penne but I haven't tried that yet so I can't vouch for it.
Coming from NY, I was pretty underwhelmed by the pizza options in HB. With that said, I would recommend A Slice of New York Pizza http://www.thebestsliceofnewyork.com/ . It has been a pretty solid choice for me when the craving hits.