Yes, GASTRULA is a Scrabble word!

Letter values in Scrabble US: G 2, A 1, S 1, T 1, R 1, U 1, L 1, A 1.

Is it a valid word?

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

double-walled stage of the embryo resulting from invagination of the blastula; the outer layer of cells is the ectoderm and the inner layer differentiates into the mesoderm and endoderm