FirstLight has 1109MW of pumped storage capacity, and 165.5MW of traditional and run-of-river hydro capacity.
Financial terms of the acquisition were not disclosed.
FirstLight is also currently building a 263.3MW natural gas peak power plant, which will bring its portfolio to 16 facilities and the combined installed capacity will be approximately 1538MW in Massachusetts and Connecticut.
In a statement, GDF Suez said the acquisition would strengthen its marketing position in the North American power and gas business, principally in the north east US and eastern Canada. A key market for the group is New England.