Missing step in Getting Started with Rails Engines: .gemspec file

The guide does not give info on the .gemspec file.

In step 4.1 we need to run bundle install, after adding the gem to the gemfile in its host app.
I got a Bundle error message: gemspec file is not valid.

And after running bundle install, my engine is not installed.

AFAIK (I am quite new to RoR) I needed to update the fields with TODO placeholders first. When I ran bundle install afterwards, it installed the engine as expected. (Yay!)

I would like to add this step to the guide, but it would be useful if someone could write a bit more to explain the .gemspec file and what (more) to do with it.

How to proceed?