My main suggestion is not upgrading yet to Angular2, is still heavily in development and you’ll face a lot of this issues like not finding a gem for rails.
Anyway, currently, angular2 can’t be compiled with sprockets (the default rails pipeline), so you really want a custom solution.
My main suggestion is go with webpack, other options are browserify or gulp (and others), that’s mainly a matter of taste. Overall, configuring a pipeline for angular2 is complicated, you have to take care of .d.ts files through typings (which is the updated version of tsd which now is deprecated), you have to transpile your typescript through tsc and possibly through babel too if you want to use async/await (which are really cool). You’ll lose the ability to reference to your files in rails like image_path and such, using a custom pipeline, so you want to take that into account too.
Typescript is much more complex to compile than simple coffeescript files, you depend on every other file it reference, since it needs to compile check against it, don’t expect something straightforward.
That being said, if you really want to work with Rails and Angular (2 or not), the right way to handle it is to have two separate projects, one with only the Rails app and one with the AngularJS path. In this way you separate concern, you can have a custom pipeline for Angular2 without impacting Rails and you’ll be forced to correctly code your Angular2 app by using Rails as a JSON API, as you should.