When I was trying to unpack ASA 8 image using GNS3 utility unpack0.1-win on my windows 7, I always received following error:

D:\ImageUnpack>unpack.exe --format ASA8 asa802-k8.bin
'gzip' is not recognized as an internal or external command,
operable program or batch file.
The system cannot find the file specified.
Traceback (most recent call last):
  File "unpack.py", line 208, in <module>
  File "unpack.py", line 146, in unpackASA8
  File "shutil.pyo", line 199, in move
  File "shutil.pyo", line 91, in copy2
  File "shutil.pyo", line 46, in copyfile
IOError: [Errno 2] No such file or directory: 'c:\\users\\palo\\appdata\\local\\
temp\\tmpk4hhmv\\vmlinuz'

Searching web I found that there is a problem with unpack utitlities, specially with pointing on the gzip util. Recommendations is to copy gzip from the folder of unpack util into \windows\system32. Then it start to work fine.