Converting a multiple-page PDF to a single image

I'm attempting to convert a PDF into a single image using GhostScript.

Only the first page is converted, while my intention is to generate a horrendously tall PNG/JPG image with all the pages concatenated together.

These are the parameters I'm currently passing to the GhostScript DLL (via a .NET application):

pdf2img \
  -dNOPAUSE \
  -dBATCH \
  -dSAFER \
  -sDEVICE=png256 \
  -dGraphicsAlphaBits=4 \
  -dTextAlphaBits=4 \
  -dDOINTERPOLATE \
  -dAlignToPixels=1 \
  -r300x300 \
  -sOutputFile=GLOSS.png \
   GLOSS.pdf

Is it possible to concatenate all the pages together via GhostScript? Or should I resort to an external method?