To install with version specification using gem install, just add the —version= option.

When you do a normal gem install, the latest version is installed.

$ gem install rails

(As of September 17, 2011, Rails-3.1.0 is the latest version)

For example, to install Rails-3.0.10, do the following:

$ gem install rails --version=3.0.10

That’s all.

【Reference】

・第1回: インストール - Ruby on Rails 3.0 日記 - Ruby on Rails with OIAX (Chapter 1: Installation - Ruby on Rails 3.0 Diary - Ruby on Rails with OIAX)

That’s all from the Gemba.