If your GDAL is from 2016 it's time to update.

Your output file format is a [GeoTIFF][1].

The default output format (in GDAL < 2.3) is a GeoTIFF.  With your GDAL, if you want an XYZ ascii file, you need to tell GDAL that by specifying the format with the `-of` flag.  

    gdal_translate -of XYZ source.grib target1.xyz

Starting with GDAL 2.3 released in 2018, if not specified, the format is guessed from the file extension. 

Note that while `gdal_translate` can output multiple bands in general, it can only output multiple bands to formats that support this. However, the [XYZ format][2] only supports a single band. So if you try to output multiple bands (with a recent GDAL) you will get the message:

    Warning 6: XYZ driver only uses the first band of the dataset.

You can of course specify a different band using a `-b` flag, but you still can only output a single band at a time to the XYZ format

    gdal_translate -b 1 -of XYZ source.grib target1.xyz
    gdal_translate -b 2 -of XYZ source.grib target2.xyz
    gdal_translate -b 3 -of XYZ source.grib target3.xyz
    gdal_translate -b 4 -of XYZ source.grib target4.xyz
    etc...
    gdal_translate -b 999 -of XYZ source.grib target999.xyz



  [1]: https://gdal.org/drivers/raster/gtiff.html
  [2]: https://gdal.org/drivers/raster/xyz.html