decompress as usual,
these are the offsets you want...
just use the patch.ff tutorial, but replace with these numbers:
Offsets Der Riese:
0002c6e7.dat
305B - FFFF
000311f1.dat
0 - 9042
TOTAL GSC LENGTH: 15FE7
If you know the offsets, you could use the patch.ff tutorial for all maps, just try to find the correct gsc