If you check the help file for G-Raster, it has a link to a page that lists a whole bunch of programs that can georeference an image and convert it into a GeoTiff or similar format that G-Raster can use directly; you cannot do this directly in G-Raster. As you've found, the GeoPDF to GeoTiff convertor only works correctly on GeoPDF files. I use PDFCreator to export PDF files in TIFF format; if you look in the options section, you can increase the dpi of the output, which might improve the output quality.
The FS raster gateway has a site with links to GeoTiffs for most of the National Forests in the US; you might take a look to see if the maps you want are there (looks like they have two maps for Shasta-Trinity). I've run several of these through G-Raster successfully.
http://fsgeodata.fs.fed.us/visitormaps/
The FS raster gateway has a site with links to GeoTiffs for most of the National Forests in the US; you might take a look to see if the maps you want are there (looks like they have two maps for Shasta-Trinity). I've run several of these through G-Raster successfully.
http://fsgeodata.fs.fed.us/visitormaps/