From Zimbra :: Wiki
Author - Sing Koo
Once you have identified the specific script file for modification, you need to decompress it using gzip. For example, if you want to alter Startup1_2_all.js, you will use the following line command:
gunzip -S zgz Startup1_2_all.js.zgz
This line command will decompress the compressed file and the name will be rename as "Startup1_2_all.js.". Please take note on the ending "." since the file has similar name, but with the dot exists in the same directory. After the decompress file is modified, you need to re-compress it using gzip. Using the same example, the command line to compress it is:
gzip -S zgz Startup1_2_all.js.
After compression is done, the compressed file with have the name "Startup1_2_all.js.zgz".