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