Thanks for the help guys, I got the .tar.xz file and it's working now.

I think the headless version is actually an tar.xz not a tar.gz. You probably need to install xz-utils. See https://www.quora.com/How-do-I-extract-and-install-a-tar-xz-file-in-Linux-Fedora-25. (Despite the link name, it also covers Debian/Ubuntu).

Edit: Not sure, but you might need to also rename ...

Hi, I'm trying to install the headless server onto a computer running Ubuntu Server.
I downloaded the .tar.gz file to /opt using wget
When I run sudo tar -xzf factorio_headless.tar.gz I get the error:
gzip: stdin: not in gzip format
tar:child returned status 1
tar: Error is not recoverable: exiting ...