Sorry I had to answer a question to ask a question so I’mma use you rq
Most likely, Gregor will open the door or someone in his family is going to force the door open. This will reveal Gregor's new form and the reader will get to see how Gregor's family reacts to his sudden transformation. Most likely they will be shocked and afraid at first.
Foreshadowing is when an author uses symbolism, an event, a small detail, or even a part of dialogue to be a warning or indication of a future event. An author might use this because they want to give the reader knowledge about a future event or to tease them.