Green’s Dictionary of Slang

1. an eccentric or senile person; spec. an aged debauchee.

1889		Barrèrre & Leland Dict. Sl. Jargon & Cant I 170/2: gaga, m. (familiar), man who, through a life of debauchery, has become almost an imbecile.
1908		I. Bloch [trans.] Sexual Life of our Time 653: In many brothels, apertures in the wall or other arrangements have been made for these voyeurs or gagas [Simes:DLSS].
1938		S. Beckett Murphy 240: Mr. Endon [...] the most biddable little gaga in the entire institution.
1941		A. Koestler Scum of Earth 180: Couldn’t understand what he said. [...] Disastrous old gaga.
1944		K. Weill letter in Symonette Speak Low (When You Speak Love) 7 Aug. (1997) 340: He is a nice old gaga, still telling the same stories about his travels in Russia.
1968		K. Brasselle Cannibals 143: Then he must be a real gagagootz for you to turn on him.

2. foolish chatter; gossip.

1945

Sun. Times (Perth) 28 Oct. 16/1: Wedding caused more society ga-ga than any similar event in Australia for years.