Grid is a term used in Surfer to describe a raster, or an interpolated ASCII/binary file which contains interpolated data on an even x,y spacing. For an XYZ grid, this looks like x and y values across the first row and down the first column, and z values filling the cells in between.
In most cases, when we've seen failure converting xyz grids, they have not actually been grid files, but have been data files with x,y,z columns of data collected at regularly-spaced intervals.
Since the file converter does not interpolate, but just converts between already-interpolated grid files, you will need to use the full Surfer program to grid the data. Once you have done so, you can use Surfer or the online file converter to convert from the Surfer GRD format to another grid file format.
Updated October 25, 2016