Excessively enthusiastic (4)
'excessively enthusiastic' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gung ho that I've seen before include "Overly enthusiastic (about war?)" , "Very (sometimes over-)enthusiastic and dedicated" , "Enthusiatic about war" , "Over-zealous to gun hog down" , "Unthinkingly eager (to fight?)" .)
(Other definitions for gaga that I've seen before include "Lady who sings" , "Slangily crazy, off one's head" , "Senile - slightly crazy" , "Doddery" , "Slang for crazy - apply a gag" .)