I made my life in RISC OS a bit simpler tonight!
I finally hacked together a stupid perl script that Filer can run to uncompress gzip files!
The perl script converts the RISC OS path like sdfs::riscospi.$.work.src.sed-src/tar/gz to a proper unixy path and feeds it to gzip -d
I don't really know perl well. I was too busy messing with awk when perl was a big thing.
Also RISC OS perl 5.8 seems broken on the pi2. 😣