Gaga is a term that immediately refers to absent mindedness. Here are some synonyms taken from a thesaurus:
Given to lighthearted silliness: empty-headed, featherbrained, flighty, frivolous, frothy, giddy, harebrained, lighthearted, scatterbrained, silly. Slang birdbrained, dizzy.
Afflicted with or exhibiting irrationality and mental unsoundness: brainsick, crazy, daft, demented, disordered, distraught, dotty, insane, lunatic, mad, maniac, maniacal, mentally ill, moonstruck, off, touched, unbalanced, unsound, wrong.