See Themes in my Cookbook for sample files in a theme.

Build

You can also omit the last argument as the filename will be inferred.

$ gem build foo.gemspec

  Successfully built RubyGem
  Name: foo
  Version: 2.5.1
  File: foo-2.5.1.gem

Unpack

$ gem unpack foo-2.5.1.gem

ls foo-2.5.1/

LICENSE        LICENSE-source README.md      _includes      _layouts       _sass          assets