A similar issue was addressed here: https://askubuntu.com/questions/796716/how-do-i-prevent-cups-from-auto-rotating-my-pdf (except that rotation was made by the filter pdftops instead of gstoraster)
There it seems the CUPS team suggested PPD file edits that solved the OP's problem. The solution however seems to have been to include the desired page size in the PPD file. In this case the paper size is already given in the stock PPD file so the solution there doesn't seem to apply here.
