It wouldn't be *illegal* to try and release it as GPL. But it would be *illogical*, nonsensical, inconsistent. One might also say that it would be impossible (because doing so, it would not really be GPL).

It's like bundling it with a license saying: "To use this software you have to pay 10 EUR per copy."
And then you offer free downloads on your webpage.

The GPL says you can't use the product with Photoshop. When you "release something as GPL", this is actually just you borrowing the GPL license and applying it to your product. If you want to make some changes or exceptions then that is fine, but then it is not really "GPL" anymore, but, say, "LeoPL" or "modified GPL".


Again: The GPL has no magical properties. It is just an example of a contract. The success of the GPL is that it has become a "standard contract" used by many people. The advantage of that is that if you read the GPL once, you don't have to read the fine print of every GPL product. The disadvantage is that some people may use it without really knowing what they are doing.


Also, note that some people question the GPL from another angle. They claim that it may not hold up in court if tested, or similar. First, this doesn't seem to be true, the GPL has been found valid in several countries (and to my knowledge, it has never been found to be invalid). Generally, courts take contracts (and thus licenses) very seriously. If you enter a contract of your own free will, courts will not listen much to later complaints that it is "unreasonable" or "silly" or whatever. There may be exceptions, in particular for consumers, when it may not be clear that someone is entering a contract.

But relying on such arguments is very dangerous, and likely to get you in trouble especially if you would end up making money with your project at some point in time. It may be argued that hobby projects are unlikely to end up in court, which is no doubt true, but on the other hand hobby projects are more reliant on people being cooperative, so it's probably a bad idea to screw people over just because they "hopefully" won't bother taking you to court.