When you originally asked about GN flat cars, I assumed that
you meant the RED CABOOSE "USRA" flat cars, not the TICHY flat
cars. My mistake.
The CHAMP decal set HC-646 was produced especially for the
GOULD (TICHY) flat cars and includes NC&StL, L&N, T&P, MKT,
GN, CP, WP and BAR -- but strangely, not SSW, which definitely
had such cars. Anyway, I've only seen evidence of the NC&StL,
T&P, SSW, and CP flat cars, even though this set was supposed
to be highly accurate.
GN had some longer straight side sill flat cars, but I've never
seen a photo of one the size of the TICHY flat.
The HC-646 contains a variety of road name lettering for GN,
so it may be good for other (and larger) GN flats than the
TICHY or RED CABOOSE kits.