to install it:

apt-get install gunzip

to untar a .txt.gz to a .txt

gunzip -c x.txt.gz >x.txt

Usage: gzip [OPTION]… [FILE]…
Compress or uncompress FILEs (by default, compress FILES in-place).

Mandatory arguments to long options are mandatory for short options too.

-c, –stdout write on standard output, keep original files unchanged
-d, –decompress decompress
-f, –force force overwrite of output file and compress links
-l, –list list compressed file contents
-n, –no-name do not save or restore the original name and time stamp
-N, –name save or restore the original name and time stamp
-q, –quiet suppress all warnings
-r, –recursive operate recursively on directories
-S, –suffix=SUF use suffix SUF on compressed files
-t, –test test compressed file integrity
-v, –verbose verbose mode
-V, –version display version number
-1, –fast compress faster
-9, –best compress better
–rsyncable Make rsync-friendly archive