Yes, GALLFLY is a Scrabble word!

Letter values in Scrabble US: G 2, A 1, L 1, L 1, F 4, L 1, Y 4.

Is it a valid word?

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

small solitary wasp that produces galls on oaks and other plants
fragile mosquito-like flies that produce galls on plants
any of various insects that deposit their eggs in plants causing galls in which the larvae feed

Also: gall wasp, cynipid wasp, cynipid gall wasp, gall midge, gall gnat.