A couple of things. First, when you initially install gems, the system automatically locks the version of the gem, whether you specify a version in the Gemfile or not. If you inherit an old application, the file Gemfile.lock will tell you what gems it is using and what versions of those gems. When you run bundle install, it will stick to those versions, even if a newer version is available.
If you want to see if there are newer versions of the gem than the ones the application is using, run "bundle outdated" as the above post indicates. That will list all of the gems used by your application that are outdated.
If you want to update a gem to a newer version, you run "bundle update [gemname]" If you don't include a gem name with the command, it will update everything (in other words, ignore the Gemfile.lock).
Hope this helps.