If you're on a Linux system you'd use gunzip and tar:
gunzip osticket_1.6.0.tar.gz
Which will give you the tar file osticket_1.6.0.tar which can be untarred using:
tar -xf osticket_1.6.0.tar
That can be done in one command but I always forget what it is. :(
If you're on Windows, 7-zip works fine (http://www.7-zip.org)