gunzip archivefile1.txt.gz
This command uncompresses the file archivefile1.txt.gz and replaces it with the uncompressed version named "archivefile1.txt".
If you want to keep the compressed version, you use the -c option, which writes to standard out, and then re-direct (">") standard out to a file like this:
gunzip -c archivefile1.txt.gz > archivefile1.txt
The ".gz" file extension is commonly used by the compression program gzip, which is the counter part of gunzip.
For all available options of the gunzip command see the man page.

