I also struggled with the same problem.
Managed to fix it by setting this parameters in ~/.zshc user profile.
You might have different RUBY_PATH, feel free to change it.
export RUBY_PATH=/usr/local/opt/ruby/bin
export PATH="$RUBY_PATH:$PATH"
export RUBY_VERSION=$($RUBY_PATH/ruby -e "puts RUBY_VERSION")
export PATH="/usr/local/lib/ruby/gems/$RUBY_VERSION/bin:$PATH"
export GEM_HOME="$HOME/.gem"
export GEM_PATH="$GEM_HOME/ruby/$RUBY_VERSION"
You can also take it from this gist