You have to compress it back then put it back in the file in the "Same Offsets"
packzip.exe -o 0x00000002 -w -15 00000002.dat _mission_recomp.gsc
With that, It will be 'compressed' back.
Then you have to fix the header and footer. Look at the first part of repacking dd.gsc in my tutorial. It is the same concept.