The house that the plane crashes into was built at one end of the only runway at Lincoln Park Airport, a very small airstrip in Lincoln Park, NJ USA (about 35 miles NW of New York City). The wrecked house was not removed for several weeks. While no planes have hit houses in the vicinity, one did bounce off the roof of a passing car several years earlier.
When Garp and his mother are in New York, they walk past some prostitutes and his mother asks why they are dressed that way. Garp tells her that they are hookers, and she says "Hookers?" but her mouth doesn't move.
When Garp and his family are playing touch football at Dog's Head Harbor, it is the afternoon, In the next scene, where Garp and Roberta are talking, the sun is shown setting over the ocean. This could not occur as Dog's Head Harbor, New Hampshire is on the east coast of the United States, so the sun should be rising.
When Garp returns home after his session with the hooker, he reminisces while opening and closing the blinds in his room. The scene of him and Helen picking up the scattered pages of his short story occurs in a different locale (spread all over the lawn) than the original event (on a road and among the landscaping near the house).
Yes, a book of the same name was published in 1980 by author John Irving.
When does Robin Williams appear as the older Garp?
Around 24 minutes into the movie.
Does Garp die at the end?
Maybe, maybe not. In the book Pooh shot him twice and he died a few minutes later in the gym. In the movie however it is not explicitly revealed, but some feel that the ending suggests he does die. He had been fascinated with flying ever since he was a child (because of his father) and with his joyous exclamation to Helen of "I'm flying!" suggests a fitting end to his life. He was able to die a happy man by having that simple childhood dream become a reality. Then again, some argue that Garp did survive as he was still alive as the movie ends and if he was seriously injured to the point where he could die at any moment the paramedics would have been actively working on him in an attempt to keep him alive instead of sitting back and allowing him to calmly speak with his wife. While medical personnel in trauma cases such as those will continue CPR and other first aid measures for a good deal of time after death, just in case the patient is able to come around, if the person is near death, they do not sit back and wait for it to happen before taking action.