Lansat in GRD and degree

convert geotiff

gdalwarp LE71920262013159ASN01_B5.TIF LE71920262013159ASN01_B5_out.TIF -t_srs "+proj=longlat +ellps=WGS84"

geotiff to grd

gdal_translate LE71920262013159ASN01_B5_out.TIF (-b 1) -of GMT LE71920262013159ASN01_B5.grd

show bands

gdalinfo

rgb to gray

gdal_translate -b 1 landsat.wgs84 -of GMT landsat.red
gdal_translate -b 2 landsat.wgs84 -of GMT landsat.green
gdal_translate -b 3 landsat.wgs84 -of GMT landsat.blue

grdmath landsat.red 255.0 DIV 0.2989 MUL landsat.green 255.0 DIV 0.5870 MUL ADD  landsat.blue 255.0 DIV 0.1140 MUL ADD 255 MUL = landsat.gray
(intensity = 0.2989*red + 0.5870*green + 0.1140*blue)