Add a working File Custom GUNZIP command

The commands under the File Custom -
Tar/Gzip
Untar/Gzip

are misleading. They are tar commands, not gzip commands

If you try and untar/Gzip on a .gz file, it says, not valid tar format.

see https://winscp.net/forum/viewtopic.php?t=28044

Use case is, many logrotate scripts gzip files. We need to easily inspect these files on occasion

gmgj wrote:

The commands under the File Custom -
Tar/Gzip
Untar/Gzip

are misleading. They are tar commands, not gzip commands

They are Tag/Gzip commands. They archive with Tar and compress with Gzip and vice versa.

I have added commands for simple Gzip to the documentation:
https://winscp.net/eng/docs/custom_commands#gzip