But looking closer, the GPL says it does not cover running the software. So, if a developer puts their code under GPL and publish the source, they could say that you are free to copy, modify and distribute that code, but if you want to run it, you need to pay a monthly fee and/or are restricted to certain use-cases or whatever.
Let's ignore the question that some versions of the GPL actually explicitly say that you can always run the software.
There are lots of other things that the GPL does not entitle you to. For example: If you buy software from IBM and buy a service contract, you can call 1-800-IBM-SERV, and they will cheerfully help you. That is true even if the software is partially or completely under GPL. If you stop paying for the annual service contract, and you try calling that number, they will (hopefully politely but a lot less cheerfully) tell you to branch to Fishkill (that is a small town in New York, and part of a bad joke about programs that crash). Just because the software is under GPL does not mean that you have a right to get support, or to get support for free, or to get precompiled binaries for free, or to run the software. It only means that the person who modifies or writes it (and holds the copyright) needs to make the source code available.
Now, I know of no single case where you need to pay to RUN GPL'ed software. Certainly, RedHat's whole business model is built around people having to pay to get (a) a convenient, precompiled, and easy to install version of the software, (b) get software where the components have been carefully vetted and aggregated into a coherent whole, (c) software that has been quality controlled and keeps getting fixed, in particular for specific hardwares and deployment scenarios, and (d) get support if it doesn't work. But you don't need to pay to run it. Matter-of-fact, the whole premise of CentOS: You get fundamentally the same software as RHEL, but without RedHat's copyrighted bits that are not under GPL, and without any paid support.
By the way, there is obviously lots of software that one needs to pay for. In many cases, the fee for the software itself is a small amount, and most of the money is for a service contract. There are even (these days rare) cases where software is distributed "for rent", with the fees proportional to how much you use it (this used to be common when licensing proportional to CPU power). There is even rare software that is closed source, but customers receive the source code once they pay for the license (and are obviously not allowed to re-distribute it). But I've never heard of someone selling the right to run software, in a case where the source code is available for free.
One big problem with this idea is that enforcement would be heinously difficult: How do you prevent people who have downloaded the source from compiling and running it clandestinely? Even if that were indeed illegal under a "pay to run" license, finding them would be darn near impossible. Another big problem would be that if anyone tried this, it would cause a massive shitstorm. Reddit and Twitter would be glowing red hot, and the seller's loss of reputation would commercially kill them (on the other hand, Reddit and Twitter glow red hot anytime Elon Musk makes more babies or does more drugs, so perhaps it wouldn't get noticed).
Now let's analyze the case where someone writes GPL'ed software, releases the source under the GPL (which clearly gives permission for other people to obtain the source and further modify it), but requires users to pay rent for using it. What prevents you from taking the source, changing one line (which you now have copyright on!), re-releasing your version, and making your version available for free (to yourself primarily)? The GPL may be viral and infectious, but only the GPL is; a modified version of the GPL (with a runtime payment requirement) would not be viral, since it states very clearly that the GPL itself (not a modified version!) attaches to any derivative work. So your new version is covered by the GPL only, not by the more restrictive license the people who wanted run-time royalties put on it.
All in all: while it's good to be paranoid (yes, only the paranoid survive ... even that is worth discussing), the probability of this scenario actually happening is either flat our zero or infinitesimally small.
So, if the bottomline of GPL is that one cannot do such unless one is rich enough to support a lawyer, well, then that's the case.
In theory, you can always be sued for any reason (or non-reason), and you may have to spend a fortune to defend yourself. If one makes that form of paranoia the guiding principle of ones life, then all one can do is to curl up in the fetal position and stop moving.
In practice, think about why someone would sue you. If you have something valuable, to get it. Well, if you have something valuable, you are by construction rich, and can afford to spend money on "insurance", like putting it into a good safe, operating a burglar alarm, and getting good advice from good lawyers. Not an interesting scenario. Second, because you pissed someone off, and under normal ethical and psychological rules they are right and you are wrong. Well, stop doing that! And if you get bankrupted for doing something bad, I personally won't shed too many tears over that. Third, if you are too poor to hire lawyers, you are also too poor to get sued, because one can't get blood from a stone. Remember, your adversary also has to pay high legal bills, and if they are acting rationally, they won't spend lots of money o lawyers and courts, if there is little chance of getting it from you. So being poor makes you safe. Finally you can be sued, because someone hates you for no logical reason, and they are crazy and irrational. Yes, this does happen (the last time it happened to me, I gave my lawyer $3K, and they went away, this was part of a political campaign, and "crazy woman" took offense to the fact that I was using quotes from her own court cases to campaign against her). But in reality, the danger from that kind of thing is minimal: If you just walk up to the judge, and explain that you are broke, but you haven't done anything wrong, and that person is crazy and vindictive, most likely the judge will understand and just close the case.
So don't worry about this. Using GPLed software is generally accepted to be safe from a legal standpoint, as long as you don't do unethical things with it.
