Gregor leaves his room for the first time after his mother sees him climbing up the walls of his room and she has a fainting spell. Grete had tried to get her out of the room before she saw Gregor, but it didn't work. Grete goes into another room to "bring some spirits or other" to "revive her mother," and Gregor follows her "as if he could give his sister some advice" like he would have done before his transformation.
When Grete sees him behind her, she startles and drops a glass bottle, and though a shard from the bottle injures Gregor, Grete grabs as many medicines as she can and runs into another room with her mother. She doesn't come out again until their father returns to the apartment and rings the bell, which Grete must answer.
Gregor leaves his room later in the story when Grete is playing the violin for the snobby boarders that the family has taken in for financial reasons. He has always loved to hear her play, and he'd even had plans to send her to the conservatory to study music before his transformation. When she begins to play, he sneaks out of his room so that he can better listen to her, and this leads to the final confrontation with his family and his death.