When you build a product with GPL or other open-source code, open your source or change the license as the license requires, and don't assume "nobody will enforce it"

Moderate value
Evidence B
Saves money
Free
Takes minutes
Some willpower

In plain words

For copyright infringement, damages are set by the other side's loss or the money you made, and in intentional and serious cases they can go up to 1 to 5 times that. One company hired a competitor's former employees and built substantially similar software, and argued that the competitor's software used GPLv2 and should have been open source. The Supreme People's Court held that whether the other side opens its source does not affect its copyright, and still ordered the company to stop infringing and pay 500,000 CNY.

Cost: Free. Before you use someone's open-source code, look at its license (the terms of use the author set out). The GPL family requires that a product using it also release its source code, which conflicts with selling closed-source software, and when it conflicts, switch to something else. The hard part is dropping a library you have gotten used to.
What you get: For copyright infringement, damages follow the rights holder's actual loss, or the money the infringer made. If neither can be worked out, the amount is set by reference to what a normal license of the work would cost. For intentional infringement with serious circumstances, damages of 1 to 5 times can be awarded. Where the public interest is harmed, illegal gains can also be confiscated and a fine imposed. Official case: Supreme People's Court (2021) Zui Gao Fa Zhi Min Zhong No. 51 ((2021)最高法知民终 51 号). The defendant hired the plaintiff's former employees and developed gateway software substantially similar to the plaintiff's software. The defendant argued that the plaintiff's software was based on GPLv2 and should have been open source anyway. The Supreme People's Court held that "whether the developer itself violates the GPLv2 license and whether it holds the copyright in the software are two independent questions," and ordered the defendant to stop infringing and pay 500,000 CNY in damages (nationwide)
Notes: This case shows that "the other side used GPL code" cannot be treated as a reason for copying it. In the other direction, there are already effective judgments in China in which a GPL author sued a commercial user who did not open source, but this guide did not find the original text on the Supreme People's Court website, so it is not cited. The grade is B because compliance with open-source licenses is governed only by general rules in contract law and copyright law, with no dedicated provision.
Evidence grade: B : Supported by research but without a precise number, or based on small or single studies.
Sources: 全国人大常委会 (2020). 著作权法（2020 年修正，第五十三、五十四条）. http://www.npc.gov.cn/npc/c2/c30834/202011/t20201119_308796.html
最高人民法院知识产权法庭 (2022). 涉「开源软件」著作权侵权案（（2021）最高法知民终 51 号）. https://ipc.court.gov.cn/zh-cn/news/view-3042.html

Translated from the Chinese original. Original book, section 11, tip 14. This section describes legal rules, mostly mainland China’s. It is general information, not legal advice; for your own case, talk to a lawyer. Content may be out of date and rules differ by place. When you actually deal with an agency, a doctor or a court, follow what local officials, doctors and lawyers tell you. Disclaimer