It is if you link it into your address space. If your code wants to be non-GPL then it needs to have some kind of barrier between it and the GPL code that it us

It is if you link it into your address space. If your code wants to be non-GPL then it needs to have some kind of barrier between it and the GPL code that it uses. For Linux, that would be the kernel syscall abi. But normally it's the process boundary. For example, if your program spawns the GNU gperf command, then its GPL license doesn't apply to your program. Furthermore, the generated code that the gperf command prints to stdout, is not encumbered by the GPL. In other words, the output of a GPL licensed program belongs to you. But if you were to copy gperf's .c files into your codebase and use their perfect hash table algorithm, then your software would become GPL encumbered, but only if you distribute. They will sue you and spare no quarter if you don't give your users the same freedoms that they gave you. Even if the gperf dev team doesn't do it, then some other org representing someone who contributed a bug fix once will. You can't hide and there is no time to survive, because GitHub can be easily monitored using tools like BigQuery.