If an app is built on a GPL Operating System must it also be GPL?
The GNU GPL license requires that if one creates a "modified version" of a "covered work" and distributes that modified version, that one must release that modified version under the GPL also. In its definitions section it provides that:
To “modify” a work means to copy from or adapt all or part of the work in a fashion requiring copyright permission, other than the making of an exact copy. The resulting work is called a “modified version” of the earlier work or a work “based on” the earlier work.
A “covered work” means either the unmodified Program or a work based on the Program.
A “modified version” is thus what is known as a derivative work in copyright law. But a work designed to interact with another work, such as an application intended to make calls to an operating system (OS) and receive services from the OS is not a modified version of the OS just because it makes such calls. Therefore, there is no need to release such an app under the GPL just because the OS is provided under the GPL.
Also note that if one creates a modified version of a GPL-licensed program for one's own use, but does not distribute or "convey" it to others, there is no obligation to place the modified work under the GPL. But the main point is that an app designed to run on an OS is not thereby a modified version of that OS.