I know I’m not really answering your question (sorry), but I honestly find the default compressor/decompressor VERY confusing, so I use ChangeV’s (http://smilebasicsource.com/page?pid=592). It has a UI, so it’s way easier to use, and it should be able to compress your GRP into a PZG file. If it doesn’t work, something might be wrong with the GRP, try to save it again with SBGED or check @BACKUP.GRP (if you have it) if this is the case.