Here's a modified version. Read the top of the script for details. The changes are basically; Made it able to run in both Poser and native Python. Make sure the drive letter and colon are not stored in the gzip header. Store the original file extension in the gzip header. Made file extension checks more stringent. Extra sanity check before deleting original file. Made path stored in gzip header begin at the first 'Runtime' encountered in path spec. You can change this easily at the bottom of the script.

I compressed a couple gig and it seems to work fine in windows.
Mac users should test it first (could someone check my logic for the Mac OS).