FWIW, My vote would be the bumper is bent, as are the mounting brackets on the right side of the picture. The left hand brackets look to be relatively straight. Neither the brackets nor the bumper look anything like a GMC or Chevy of your vintage.
Below is a snapshot of a reproduction 1951 Chevy front and rear bumper set with brackets. The GMC ones were to my knowledge the same. One of our fellow long time GMC owners will hopefully chime in with more details.
