Yes, GAMMAS is a Scrabble word!

Letter values in Scrabble US: G 2, A 1, M 3, M 3, A 1, S 1.

Is it a valid word?

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

the 3rd letter of the Greek alphabet
a unit of magnetic field strength equal to one-hundred-thousandth of an oersted
Portuguese navigator who led an expedition around the Cape of Good Hope in 1497; he sighted and named Natal on Christmas Day before crossing the Indian Ocean (1469-1524)

Also: da gamma, vasco da gamma.