Yes, TONGA is a Scrabble word!

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

Is it a valid word?

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

a monarchy on a Polynesian archipelago in the South Pacific; achieved independence from the United Kingdom in 1970
the language of the Tongan people of south central Africa (Zambia and Rhodesia)

Also: kingdom of tonga, friendly islands.