The Ruby License and the License of Ruby are two different things.

The Ruby License is the license which Matz wrote for Ruby.  The license
of the ruby distribution itself is a disjunctive license which gives
the user a choice of distributing under either of the two licenses.

You are free to follow the terms of either license.  For your own
applications, if you choose to follow the restrictions of the GPL, you
need to ensure you use a GPL compatible license, but it needn't
necessarily be the GPL.  The various MIT / BSD style licenses are
mostly GPL compatible, as are many other license.  A full list of GPL
compatible licenses according to the Free Software foundation can be
found here:

http://www.fsf.org/licensing/licenses/index_html#GPLCompatibleLicenses

If you are not planning on using GPLed libraries or other things under
the restrictions of the GPL, under the Ruby license you can basically
use whatever licensing you'd like, but bear in mind that the Free
Software Foundation does not consider the Ruby license on it's own as
free software, therefore you will prevent people from linking your code
with GPLed code if you use just the ruby license or some other non-free
license.

This is indeed quite similar to the License of Perl, which offers a
choice of the Artistic License or the GPL.

As a side note, the Ruby community seems to have a tradition towards
the BSD style licenses.  My role as a free software advocate is to
whine about the fact that these licenses do not offer a strong
copyleft.  However, it seems like such a thing is exactly what you'd
like to avoid, so these licenses might fit your needs well as well as
provide some freedom to your work.

The compromise that will make most Rubyists willing to use your
software regardless of their politics is to offer a disjunctive license
such as the License of Ruby (Ruby License / GPL) or any other mix and
match choice that allows the GPL as an option.  However, I'm sure some
of the Gurus will say, just BSD the sucker!, and I will try not to make
my case for the FSF too strongly.

I work on commercial code under the License of Ruby, and have not run
into any issues with the scheme, if that's of any help.

Hope this helps.
-Greg