there is no "non-compressed" option, i believe. I searched the documentation and the UI, no way. but you are no completely off, good comparison tools have so called 'converters', or even support un-gzipping on the fly when comparing.
The tool I work for, ECMerge, has a gzip converter, you just need gzip installed in your system path (or specify it by hand in the configuration screen), you will need to add *.gnumeric to the supported pattern for GZip converter and ECMerge will diff the gnumeric files content. There is no 'gzipped merge' though.