"Pirated" is a bit of a vague term, that can lead to a lot of FUD (fear, uncertainty and doubt). It can mean pretty much any number of things, but is quite often used for anything that is a copyright violation.
If that's the definition here, the answer is trivial through tautology. If the software is used in violation of copyright, then yes, it's a copyright violation to use it. Since that rather obvious, I'll assume that's not what your question is. Let's explore some options.
First off, let's dispel some common misconceptions. If someone makes a derivative work of a GPL licensed work, and they distribute it, they legally must do so under the GPL. A common mistake is that people think this means that if they distribute it under another license, they may use it under the terms of the GPL. This is not true. If they violate copyright by not releasing their software under the GPL, this does not automatically make their work GPL as well (but rather, legally undistributable).
This means that the question of whether the creators of the software are legally allowed to distribute it under a non-GPL license is no factor in your question. If you believe this is indeed the case, you could see if you can do something about GPL enforcement (see: What can/should I do when I see a violation to GPL restrictions?). The question on whether Drupal themes are required to be GPL is not in scope of this question (but would make a good and on-topic question).
If they chose to distribute their software under some license other than the GPL, you may not use the software under the GPL. If you received the software from someone who claimed it was the GPL, and you use it in good faith that it is actually GPL, and afterwards it turns out it is not GPL, you will still have to abide by the actual licensing terms. You will not be in legal trouble for willful infringement, but you can't continue to use the software under the false terms that were offered to you.
I can't estimate whether a court in your situation would find that you were acting in good faith or not, but I'd imagine this is not a situation you want to find yourself in at all.
It is however very plausible that the copyright holders of the software sell the software to others under the GPL. These others then may further re-distribute the software without any required fee. If this is the case, then yes, you can use the software under the GPL.
In their FAQ, the FSF has an answer that amounts to that if the software is distributed to anyone under the GPL, and you receive the software from anyone claiming it is under the GPL, you may use it under the GPL whether you can prove the chain of correct license transfers or not. They're not very definitive about it, but this doesn't seem to be a big problem.