Yes, GUAR is a Scrabble word!

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

Is it a valid word?

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

drought-tolerant herb grown for forage and for its seed which yield a gum used as a thickening agent or sizing material

Also: cluster bean, cyamopsis tetragonolobus, cyamopsis psoraloides.