I've had good and bad gizzards, more bad or mediocre than good.I've done chicken offal ala sous vide several times and settled on 165 at 18 to 20 hrs as the magic time/temp. No need to even cut the white out of the gizzards they are tender. Hearts and livers are creamy smooth in texture and gizzards are like fillet mignon.
Chile, I prefer the hearts (little nuggets from the Gods) but don't discount the gizzards. Done right they are better than high end steak.
Wish I had access to a truckload of turkey gizzards... I throw the offal in the oven/smoker when doing turkey.. they are the cooks snack.... along with the Pope's nose and neck...