An 8-year-old boy has died of injuries suffered when a goal post fell on top of him at the South Mountain YMCA in Phoenix.Gabriel Mendoza died at St. Joseph's Hospital within hours of the accident Thursday.Investigators said Mendoza was playing goalie and grabbed the overhead bar to swing from it when it came crashing down. The impact killed him.He was transported to the hospital where he died at about 6:30 p.m.