Server Gem Listen
Automatically restart Rails server after changes in gem sources. If you want to watch for changes in local gems you may want to use lightweight alternative require_reloader.
Warning: this gem not for actual development or production usage. You can use it for quick 'scientific' experiments with core or something like this. I would recommend setting up an isolated environment using something from rbenv/docker/vagrant.

Installation
gem install server-gem-listen
Usage
Execute the command below under your project root:
server-gem-listen
This will launch rails s wrapped with gem watcher.
License
Server Gem Listen is released under the MIT License.