For now {Gem} isn't released on RubyGems. For now you could download a GitHub release. Or compile it from source:
$ gem build {Gem}.gemspec
Building RubyGem...
Name: {Gem}
Version: {Version}
File: {Gem}-{Version}.gem
$ gem install ./{Gem}-{Version}.gem
Installing... {Gem}-{Version}... Done...
1 gem installed