Yes, GILLS is a Scrabble word!

Letter values in Scrabble US: G 2, I 1, L 1, L 1, S 1.

Is it a valid word?

GILLS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a British imperial capacity unit (liquid or dry) equal to 5 fluid ounces or 142.066 cubic centimeters
a United States liquid unit equal to 4 fluid ounces
any of the radiating leaflike spore-producing structures on the underside of the cap of a mushroom or similar fungus
respiratory organ of aquatic animals that breathe oxygen dissolved in water