This snippet can be used to uninstall all the Gems in your machine except the ones specified in specific Gemfile.locks. Answer to SO question http://stackoverflow.com/a/16479097/465082

gem_cleaner.rb

require 'bundler'

# Required for Bundler::LockfileParser. This can be empty though.

`touch Gemfile` unless File.exists?("Gemfile")


# Add the paths to the Gemfile.lock files, the gems in which 
# need to be protected

dot_lockfiles = [
  # Add the dot_file paths you need.
  "/path/to/gemfile1.lock",
  "/path/to/gemfile2.lock"
  # ..and so on...
]

# As the name suggests, the LockfileParser class parses a .lock file
# and generates a parse-tree of Gem specifications, Gem dependencies, Gem sources,
# and Gem platforms.
# These in turn can be obtained by calling #specs, #dependencies, #sources, #platforms etc. 
# on the parsed lockfile object.

lockfile_parser = ->(path) do
  Bundler::LockfileParser.new(File.read(path))
end

# The #specs method for a Bundler::LockfileParser object
# generates an array of the gem specifications of all the gems
# that are a part of your bundle (the Gemfile). The output of 
# the #to_s method is in this form: 'zurb-foundation (4.1.6)'

lockfile_specs = ->(lockfile) do
  lockfile.specs.map(&:to_s)
end

# We will use this function to remove the parentheses, commas in a string so that
# string of the form 'zurb-foundation (4.1.1)' can be cleaned up. This returns an Array
# in the form ['zurb-foundation', '4.1.1']

split_and_de_parenthesize = ->(string) do
  string.split(" ").map {|x| x.gsub(/\,|\(|\)/, "")}
end

# CAREFUL!!11!!1
# Uninstall command. Replace the #puts function with `gem uninstall #{..the code..}`
uninstaller = ->(string) do
  puts string.map(&split_and_de_parenthesize).join(" -v ")
end


# For lack of a better name, I'm using this.
# The #splitter method converts a gem list string in the form 'zurb-foundation (4.1.1, 4.1.6)'
# into two parts and returns an array in the form:
# ['zurb-foundation (4.1.1)', 'zurb-foundation (4.1.6)']

splitter = ->(string) do
  # Hattip to @jessitron for suggesting the pattern matching style.
  (gem_name, *versions) = string.map(&split_and_de_parenthesize) 
  versions.map {|x| "#{gem_name} (#{x})"}
end

# The #lazy method is available only in Ruby 2.0 and greater versions. If you're on < Ruby 2.0.0,
# remove the #lazy method and the #to_a method at the end.
gems_to_be_kept    = dot_lockfiles.lazy.map(&lockfile_parser).map(&lockfile_specs).to_a.uniq

all_installed_gems = `gem list`.split("\n").map(&splitter).flatten

gems_to_be_uninstalled = all_installed_gems - gems_to_be_kept

gems_to_be_uninstalled.map(&uninstaller)