a) Give `gunzip` the `--keep` option (version 1.6 or later)
> -k --keep
> > Keep (don't delete) input files during compression or decompression.

 gunzip -k file.gz

b) pass the file to gunzip as stdin

 gunzip < file.gz > file

c) use `zcat` (or, on older systems, `gzcat`)

 zcat file.gz > file