I'd suggest to get more information about the repercussions associated with appropriating GPL code into proprietary closed source.
This is a big deal. You may have to license your entire codebase under GPL if you incorporate GPL code and distribute it.
I would suggest that you actually take your own advice and get more information yourself.
No license can force you to release your code. Nope, not even GPL.
Instead, what a rights holder can do, is sue for damages for the copyright theft, for not following the license. They can't force you to follow the license. Instead, they can say that you didn't follow it, therefore you stole the code, and owe money to them, for stealing the code, depending on how much the code is worth.
The only thing that GPL does, is it gives people permission to use the works, in exchange for releasing code. But, if you infringe, the damages do not depend on whatever the license was, or whatever request the license makes.
To use an example someone else gave, of the "first born child" license, imagine someone writes a simple binary search function, and puts out a license that gives it out for free, in exchange for paying them some absurd price. EX: the joke of the first born child, but more seriously, lets say the license was "1 million dollars".
If someone stole that binary search, couple line function code, and it went to court, they absolutely would not own them 1 million dollars, even though thats what the license said.
Instead, they would owe the rights holders damages. And chances are, a couple line binary search function, or some other example that you could think of, would only be worth a small amount.
And even though the license said "This code is worth 1 million dollars, and you owe us that money if you use it!", it is not true that anyone would owe them a million dollars. Instead they would only owe them damages, which would not be anywhere close to 1 million dollars.
In most cases, damages are set to make both parties straight, not to be punitive. People cite how trillion dollar companies might have billion-dollar lawsuits, but that's pretty reasonable. $1B damages are 0.1% of a company's value in a battle between FAANGs, which have big-O trillion-dollar valuations. If you have a dispute between $1M businesses, the analogue is $1k damages. That's not atypical for a commercial dispute.
I did not say it forces you to distribute it. That's absurd.
What I said is: "if you incorporate GPL code and distribute it"
If you do those two things, yes, you have to license your code under GPL.
It's not me saying, please take a look at Section 5-b and 5-c of the license. [1]
Let's do an experiment: You need to hit yourself repeatedly in the head with a mallet until you pass out.
Are you currently hitting yourself with a mallet until you pass out? No. Just because something is written doesn't mean you need to do it. If I incorporate your GPL code, distribute it, and don't license my code under the GPL, that means I'm distributing code without a license (or breaking a license). Unless I've crossed the line for criminal prosecution (which is far from anything we're discussing here), the worst-case consequence of that is .... damages.
If I've crossed the line into criminal prosecution, then the consequence is damages and jail time. I absolutely STILL do not need to license my code under the GPL.
(In most cases, it's a good idea to license code under the GPL, though, both due to branding/reputation damage, and since usually that leads to an out-of-court settlement; but those carry no legal force being that)
