does using ":platforms =>" in your gemfile work?

Rails 5:

if Gem.win_platform?
  # Install gem for Windows
else
  # Install another gem
end

Bundler concept of platform differs from normal understanding of RUBY_PLATFORM matching or RubyGems behaviors.

You can find the entire documentation about how to use platforms for Bundler here:

http://bundler.io/v1.14/man/gemfile.5.html

You might not need therubyraceron Windows (it actually doesn't work), but you might need execjs so CoffeeScript or other details of Asset Pipeline work properly

In your case, I will do:

gem "execjs"
gem "therubyracer", :platforms => :ruby

UPDATE: execjs gem might be installed because another dependency (not limited by platforms) is depending on it to be installed.

Add code to the Gemfile like this that excludes/includes gems depending on the OS platform

if RUBY_PLATFORM=~ /win32/ 
   gem "windows-only-gem"
else
   gem "os-agnostic-gem"
end

:platforms => :ruby does indeed exclude gems from being installed on Windows.

However, it does not work in a cygwin environment. In cygwin, it considers the platform to be :mri.

You'll also notice that ruby -e 'puts RUBY_PLATFORM' outputs i386-cygwin, not i386-mingw32 or i386-mswin like it would on Windows ruby.

Were you working in a cygwin environment?