Meanings of red gills in English

Fish are good if they are firm, have clear eyes, and red gills.

Simone's red gills winced, like a dying fish, but he was too drunk to qualify.

You can tell fresh fish by its clear eyes, red gills, firm appearance, and ocean fresh smell.

Sometimes, codfish, though exhibiting signs of rough usage, will eat much better than those with red gills, so strongly recommended by many cookery-books.