From the beginning, I tried to import the MODFLOW files into GMS by opening the .nam file, as you suggest and as recommended in the wiki. The good news is that I can now open the files in GMS. The issues were a minor format error in the .dis file (0s were in a column and GMS appears to require them in a row) and a format error in the .rch file (there were single quote marks (') that were unacceptable in GMS). Tech support solved that problem. The bad news is that even though GMS can read the files, MODFLOW cannot be run and it comes up with the message "Error reading array". The bottom line is that GMS is not compatible with a basic MODFLOW model produced by the USGS without some tinkering.