unzip decompress zip algorithm same *.zip files and gunzip decompress gz algorithm that same *.gz files.
If you migrated from windows, You have many compression algorithm in UNIX world such as *.7z , *.gz , *.bz2 and so on. Also you can compress file which already archived with tar command:
such as :
tar -zcvf mydir.tar.gz mydir #gz and tar
tar -jcvf mydir.tar.bz2 mydir #bz2 and tar