Step 1. In the command line navigate to the folder containing your '*.asc' files.
Step 2. Type the following: gdalwarp --config GDAL_CACHEMAX 8000 -wm 8000 *.asc mosaic.tif
The processing of each file should follow this format on the command line as above. Job Done. An image entitled 'mosaic.tif' will have been created in the same directory.
Step 3. Simply import the 'mosaic.tif' into Grass GIS. Apply your own colour table as necessary.