As you looked up on the dictionary, 가만하다 means to be quiet but this word is usually used like 가만히 which means to be quiet without moving. People just take the word as the meaning of the situation you are not moving. E.g. 가만히 쳐다보다 i’m staring it without moving (quietly) 가만히 앉아 있다. I’m sitting on without moving. 가만히 공부만 했다. I only studied without moving.