Yes, GIANTS is a Scrabble word!

Letter values in Scrabble US: G 2, I 1, A 1, N 1, T 1, S 1.

Is it a valid word?

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

any creature of exceptional size
a person of exceptional importance and reputation
an unusually large enterprise
a very large person; impressive in size or qualities
someone or something that is abnormally large and powerful
an imaginary figure of superhuman size and strength; appears in folklore and fairy tales
a very bright star of large diameter and low density (relative to the Sun)

Also: giant star.