An onomatopoeic expression for chattering (of teeth) and trembling from fear, cold, or nervousness; often used to emphasize shaking.
夜中の冷たい風で足がガクガクブルブル震えて眠れなかった。
My feet were shivering (chattering and trembling) from the cold wind at night and I couldn't sleep.
彼は怖くてガクガクブルブルして、声も出せなかった。
He was so frightened that he was trembling and couldn't even make a sound.
寒さ対策をせずに山に行ったら、ずっとガクガクブルブルしていた。
I went to the mountains without proper cold protection and kept shivering the whole time.