Yes, GAMBIT is a Scrabble word!

Letter values in Scrabble US: G 2, A 1, M 3, B 3, I 1, T 1.

Is it a valid word?

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

an opening remark intended to secure an advantage for the speaker
a maneuver in a game or conversation
a chess move early in the game in which the player sacrifices minor pieces in order to obtain an advantageous position