고 놀라다

be surprised at seeing or hearing

The -고 clause gives what caused the surprise, and here it is usually something seen: 크기 보고 놀람 "shocked by the size", 사진을 보고 놀라는 멤버들 "the members startled by the photo", 제 꼬라지 보고 놀라신 거 아니죠? "you weren't shocked by the state of me, were you?". 그 얘기 듣고 놀랐어 is being surprised at what you heard. 놀라다 is being startled or amazed, not frightened — for fright you want 무섭다 or 겁나다.