You can write any conditions you want into a license.
That's what actually differentiates proprietary licenses from open-source licenses.
Open-source licenses follow certain rules, and you usually select an existing license, so therefore they can be reasoned about, collectively. People often implicitly mean "OSI-approved license", when they talk of "open-source licenses".
Proprietary licenses, on the other hand, can contain whatever bullcrap you want.
Having said that, I'm not a lawyer, but I imagine, if you also called your license "GNU General Public License", then a case could probably be made in court, that your license is deliberately confusing.
Oracle Java uses a license called "GPL with Classpath Exception", so it's definitely possible to create derivatives of the GPL and name them appropriately.
Wow, I've definitely seen that before, but I never realized how wild that is. So many companies will start drooling like a dumbass when anything contains the GPL.
So, it's not like they can't ever use GPL software, most do use Linux knowingly or unknowingly. But if you use GPL software in a way the legal department hasn't seen before, they'll always feel uneasy about it.
Frankly, I'm surprised that Java gained any traction in the corporate world at all, then.
Contracts need to follow the set of rules in your country that dictate what can and can't be enforced. It's an entire branch of law.
If you try and pull something like OP is suggesting, the worst case scenario is that it may render the contract an unlawful document and therefore void.
I imagine in most places things like what OP is suggesting would get laughed out of court.
You can have some fun though. I heard of one guy who, as a recruitment bonus, insisted that it be included in his contract that he receive an office desk made entirely of Lego.
I just like to interject for a moment. What you're referring to as butthole is in fact GNU/butthole, or as I've recently taken to calling it, GNU+butthole.
You can write any term you want for your software. There have been instances of people adding terms to their licensing like "You hereby accept to forfeit your soul's ownership to the creator of this software."
The GPL states that its text must not be modified. I take this to mean (though I'm no lawboy), that if you wanted to have a license with the same terms plus some changes (the butthole rule), you would have to rewrite the whole thing. It would not nearly be a "GPL license".
This is very important nitpicking I'm doing here, okay?
Nah, there is the class path exception which is like an amendment to the license sort of. It doesn't change the text of the license, it's just added at the end. Also, nothing is stopping you from dual licensing. In the same way many companies provide a version of their software under GPL but provide a version under an exclusive license if you pay them, you could give people the code under an exclusive license if they send a butt pic.