because if those Rails versions were not written with 1.9.2 in mind, then there might be slight usage difference that can break Rails.
Actually, can any gem command tell a particular gem is dependent on what versions of other gems (and perhaps Ruby version too)?
Gem dependencies on other gem versions can be seen at http://rubygems.org , though it doesn't mention the dependencies of dependencies.
As for ruby versions, I think the README is your best bet - other questions on Stack Overflow have asked if it's possible to put that metadata into a gem, and you can't really.
Also, don't forget http://isitruby19.com
From the ruby guides:
Rails 2.3 should pass all of its own tests whether you are running on Ruby 1.8 or the now-released Ruby 1.9.1. You should be aware, though, that moving to 1.9.1 entails checking all of the data adapters, plugins, and other code that you depend on for Ruby 1.9.1 compatibility, as well as Rails core.
Which assuming the tests are comprehensive, implies rails => yes, other gems & adapters => maybe
On your other question, the command:
gem dependency <gem name>
can be used to list all the dependancies a particular gem has, see documentation here for details. However it doesn't list any dependency on ruby version.
This topic came up on the rubyonrails-core mailing list earlier today. The short version is that it's currently untested, and there are bound to be some compatibility issues. As I said on that thread, rails 2.3.x corresponded to the betas / development releases of ruby 1.9.1, and by the time our release date arrived 1.9 was still too unstable to justify the effort.
1.9.2 is pretty reliable on the whole, but until someone (possibly you) does some thorough testing, it's not really safe to assume that your application will work. Keep an eye out for future 2.3.x point releases as they may add compatibility.