gdal_merge.bat -co COMPRESS=PACKBITS -of GMT -o "C:/Users/hamp/Documents/BC & AK/NOAA RNC ALaska/large area charts/testmosaic.tif" "C:\Users\hamp\Documents\BC & AK\NOAA RNC ALaska\large area charts\Grouped` `Files\17400_1.KAP" "C:\Users\hamp\Documents\BC & AK\NOAA RNC `ALaska\large area charts\Grouped Files\17420_1.KAP"`

Format driver GMT does not support creation and piecewise writing.
 Please select a format that does, such as GTiff (the default) or HFA (Erdas Imagine).