gunzip all .gz files in directory

How about just this?

$ gunzip *.txt.gz

gunzip will create a gunzipped file without the .gz suffix and remove the original file by default (see below for details). *.txt.gz will be expanded by your shell to all the files matching.

This last bit can get you into trouble if it expands to a very long list of files. In that case, try using find and -exec to do the job for you.

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From the man page gzip(1):

gunzip takes a list of files on its command line and  replaces  each  file
whose  name  ends  with  .gz, -gz, .z, -z, or _z (ignoring case) and which
begins with the correct magic number with an uncompressed file without the
original  extension.

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Note about 'original name'

gzip can store and restore the filename used at compression time. Even if you rename the compressed file, you can be surprised to find out it restores to the original name again.

From the gzip manpage:

By default, gzip keeps the original file name and timestamp in the compressed file. These are used when decompressing the file with the -N option. This is useful when the compressed file name was truncated or when the time stamp was not preserved after a file transfer.

And these file names stored in metadata can also be viewed with file:

$ echo "foo" > myfile_orig
$ gzip myfile_orig 
$ mv myfile_orig.gz myfile_new.gz 
$ file myfile_new.gz 
myfile_new.gz: gzip compressed data, was "myfile_orig", last modified: Mon Aug  5 08:46:39 2019, from Unix
$ gunzip myfile_new.gz        # gunzip without -N
$ ls myfile_*
myfile_new

$ rm myfile_*
$ echo "foo" > myfile_orig
$ gzip myfile_orig
$ mv myfile_orig.gz myfile_new.gz 
# gunzip with -N
$ gunzip -N myfile_new.gz     # gunzip with -N
$ ls myfile_*
myfile_orig

Use this command to gunzip (unzip gz files) all files in the current directory and keep the original ones:

gunzip -k *.gz