I\'ve been using ghostscript to do pdf to image generation of a single page from the pdf. Now I need to be able to pull multiple pages from the pdf and produce a long vertical
First check the output device options; but I don't think there's an option for that.
Most probably you'll need to do some imposition yourself, either make GhostScript do it (you'll have to write a PostScript program), or stitch the resulting rendered pages with ImageMagick or something similar.
If you want to try the PostScript route (likely to be the most efficient), check the N-up examples included in the GhostScript package.