How to see the dependency tree just from Gemfile?

gem dependency (with no args) should show you all gems from current Gemfile with their dependencies.

Edit:

You can also do gem dependency -R (or just dep instead of dependency) if you want to find out which gems use specific (or all) gems.

For deeper dependencies I'd parse output (regex maybe?) of first gem dependencies, pick gem's names and call gem dep on each of them, but that's just a loose idea.

You can also use bundler to create a dependency graph.

Install graphviz:

gem install ruby-graphviz

and then:

bundle viz

Here are an example of a newly created Rails app:

You can also play with the options:

bundle help viz