I have Ruby installed, but I still need to add it to the PATH variable. I found something online for how to manually add it using the command line:
set PATH=
I know this is questions has the Windows tag, however it is one of the first DuckDuckGo results for "ruby gems add to path" so I just wanted to add this.
On Linux you can add this line to the end of your .bashrc in order to add it to your path:
.bashrc
export PATH=$PATH:~/.gem/ruby/2.6.0/bin