If you do a
on either file it should tell you the compression used then you can decompress like that.
I think when i looked at initrds on fedora core it was cpio compression so thats a possibility.
edit: I just checked mine now and the commands needed were
cp /boot/initrd-blah .
mv initrd-blah initrd-blah.gz
gzip -d initrd-blah.gz
cpio -i < initrd-blah
Those will extract the contents into the current directory so i would advise doing it in a new empty directory.