surprised, frightened
愕 centers on a sudden, strong emotional shock: being surprised or frightened by something unexpected.
愕 combines the heart radical 忄 with 咢, which likely contributes the sound and may relate to a sudden outcry or shock. The exact historical development of 咢 is uncertain.
The heart radical 忄 beside 咢 suggests a heart jolted by a sudden sound or event. Picture your heart leaping when you hear a loud, unexpected noise: that's the surprise and fright of 愕.
For ガク, imagine a sudden shock making you gasp 'Gah!' and then you're stuck in place: Gah + stuck -> ガク. The heart 忄 is startled by the loud 咢 sound.