You pay for all the software you use, unless you are in a country with public domain and the software is clearly cast into it.

Some software you pay for in dollars. Some in postcards. Some by exchanging patent rights, or promising not to sue. Some by accepting contracts to contribute back changes you make, under certain circumstances.

There is no legal jeopardy involved in using GPL software that is different from buying a math library: you read the contract, you pay the price, you use the software in accordance with the contract.

Violate the contract, penalties attach, starting with enforcement of the contract.