Yes, GESTATIONS is a Scrabble word!

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

Is it a valid word?

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

the period during which an embryo develops (about 266 days in humans)
the state of being pregnant; the period from conception to birth when a woman carries a developing fetus in her uterus
the conception and development of an idea or plan

Also: gestation period.