System-wide config of gem operations? The answer to just about everything gem related on El Capitan -- including installing fastlane -- for me has been using this switch -n /usr/local/bin

(So sudo gem install fastlane fails but sudo gem -n /usr/local/bin install fastlane works.)

How can I apply this to all gem operations automatically?
Because El Capitan introduction to SIP (System Integrity Protection) you need to change your GEM_HOME or do similar something like sudo gem install bundler -n /usr/local/bin.