macOS does not come with an FTP command line tool. You can connect **read-only** (just for downloading things) to FTP servers using Finder. Open Finder and choose Go → Connect to Server (⌘K or <sub><img src="https://i.sstatic.net/VhQpf.png" width="50"></sub>) and enter `ftp://ftp.gnu.org/`.

<img src="https://i.sstatic.net/XnNT7.png" width="399">

When prompted for a username and password, choose Guest if the server does not require authentication, which is the case with ftp.gnu.org.

<img src="https://i.sstatic.net/2khUQ.png" width="430">

If you do need ftp on the command line, you can install ftp using [Homebrew](https://brew.sh): 

    brew install inetutils