1. Download GNS normally from the website (download the .dmg and install it).

2. Try running telnet from your terminal, if it runs okay, skip this step. If it doesn't (If you have MAC High Sierra, Telnet has been removed, so you need to install it.)

According to this article, do the following to re-install telnet

This uses version 1.9.4, if you use this check for newer versions.

If you don't have xcode and you want to use Homebrew for the installation, install it first:

xcode-select --install

To install telnet, execute these commands: Source: https://dor.ky/restore-telnet-in-mac-os-high-sierra-10-13/ Make sure to use the latest version of telnet the one below is 1.9.4

curl http://ftp.gnu.org/gnu/inetutils/inetutils-1.9.4.tar.gz -o inetutils-1.9.4.tar.gz
tar xvzf inetutils-1.9.4.tar.gz
cd inetutils-1.9.4
./configure
make
sudo make install