IANAL, but I used to redistribute Free Software...
In general, the answer is that (a) you own what you wrote
and (b) using Ruby or Rails libraries doesn't "infect" your
code in any manner. Static linking and inclusion of source
code are exceptions, but neither tends to be a problem. Use
of an interpreter or the supporting libraries is well-trodden
ground and isn't a problem.
Open Source licences aren't supposed to restrict the way in
which the code is used, only its redistribution, relicensing,
etc. So, as long as your use is within (say) a company, you
aren't likely to have any problem at all. Even when you are
redistributing software externally, the rules aren't onerous:
* Don't claim that you wrote or own others' material.
* Don't distribute binaries that are based on GPLed code
without being willing to distribute the source code, as
well. This could, for example, apply to distributing
the Ruby interpreter.
However, the "Library" (aka Lesser) GPL was specifically
designed to handle this situation. In the unlikely event
that the regular GPL was used AND the author is unwilling
to accept the Lesser GPL AND redistribution is an issue,
you might have a problem. But it's unlikely...
In any case, accommodating and courteous behavior will get you
out of most disputes. Conversely, strict adherence to legalisms
may be insufficient to avoid calumny, if you aren't perceived as
being courteous and acting in good faith.