Is there any difference between specifying exact versions of third-party libraries in Gemfile and using Gemfile.lock?
If I use exact versions in Gemfile will it be the same as distributing Gemfile.lock?
No, an exactly specified Gemfile and using a Gemfile.lock is not the same.
Your Gemfile might include all gems you are using with a specific version. But the Gemfile.lock will also include all gems that are dependencies of the gem you use. That means that a typical Gemfile.lock will include way more gems when a Gemfile.
Furthermore: You might have gems or gem versions to your Gemfile that are incompatible with each other. A Gemfile.lock is generated by bundler and represents a set of gem versions that are compatible with each other. If bundler is not able to fulfill all required dependencies then it will not generate a Gemfile.lock.
That said: Pin only versions in your Gemfile that you need to pin because of version requirements of your app. Let bundler find a valid combination and check that Gemfile.lock into version control system.