After ignoring the advice to use Ruby 1.8.2 instead of the more recent 1.8.3, I got the "uninitialized constant Mysql" error and decided to downgrade. With Gentoo, this was very easy:
sudo emerge -ua =ruby-1.8.2
ruby --version # just to verify correct downgrade.
But this didn't solve the "unitialized constant Mysql" error. So even now I do not know what, if any, problems this solved.
I did solve the problem by applying a suggestion I found on a Gentoo forum:
sudo emerge -a mysql-ruby
Apparently somehow the previously emerged mysql-ruby package was corrupt or something, and re-emerging it was all it took to fix it.
Ruby on Rails is a very amazing framework for writing web applications fast. If I were a Mac developer, I would use Ruby and Rails. Since I'm a Windows developer, I still use ASP.NET, primarily.