The General Public License doesn't always force you to make your code available

I just answered this question on stackoverflow asking for "ways around the GPL". While the motives of the poster seem questionable, this did prompt me to explain a subtlety of the General Public License which he may not have understood: the GPL does not automatically force you to release your code as soon as you use GPL'ed code.

The GPL only requires that you make the code available to anyone you distribute the software to. This is not exactly the same as releasing it to the whole world for free. From the GPL FAQ:

Does the GPL require that source code of modified versions be posted to the public?

The GPL does not require you to release your modified version, or any part of it. You are free to make modifications and use them privately, without ever releasing them. This applies to organizations (including companies), too; an organization can make a modified version and use it
internally without ever releasing it outside the organization.

But if you release the modified version to the public in some way, the GPL requires you to make the modified source code available to the program'susers, under the GPL.

Thus, the GPL gives permission to release the modified program in certain ways, and not in other ways; but the decision of whether to release it is up to you.

So if you use GPL code for building an internal tool which you will never distribute to third parties, then there is no requirement to distribute the source to anyone.

Interestingly enough, the above subtlety also applies if you only make the software available as a web application hosted on your own web servers. Since you aren't technically distributing the application to the users, you don't have to give them the code. The Affero General Public License (AGPL) was designed specifically to close this loophole.