1. 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

2. Pass the file to `gunzip` as stdin

 gunzip < file.gz > file

3. Use `zcat` (or, on older systems, `gzcat`)

 zcat file.gz > file