Nagging; complaining repeatedly — an onomatopoeia for scolding or grumbling at length.
母は毎朝ガミガミと子供を叱る。
Every morning my mother nags at the children.
社長はミスがあるとすぐガミガミ言う。
The boss immediately starts nagging when there's a mistake.
彼女はガミガミ言うタイプではない。
She is not the type to nag.