I suspect that there exists a gdal tool (gdal_translate?) that will convert a 4-channel PNG (+ world file) to a 1-channel (gray-scale) geoTIFF. From the gdal_translate man page, it appears simple to go in the other direction, i.e. to expand a monochrome raster to a 3 or 4-channel image, but I can’t figure out how to go from many channels to one.
If not gdal (hardly conceivable), then another *nix command-line tool?