You've been able to do that since version 8. Only problem is that the way GM reads it doesn't completely agree with MP_TYPE assignments. So, in order to avoid lots of "unknown point type" warnings at the beginning of a cgpsmapper compile I import the GNIS file into Excel and use a vlookup table to assign the correct MP_TYPES.
Another problem with GNIS files is they change the format, column headers, column delineators, from time to time. At least twice in the past three years. IIRC the old spacer was a tab. Now it's a "|".