I don't think mere bundling constitutes derivation. For example, could I distribute a CD-ROM that includes both GPL and proprietary binaries? What about if it also included a script that calls both GPL and proprietary binaries? Is that script GPL?
As I understand it, the main test for GPL applicability is, roughly speaking, "does the GPL code run in the same address space as the non-GPL code?" Just seeing that some code has been included in a binary is not enough, because we don't know how that information is going to be used. For example, is it going to be copied to the file system and then executed normally? Is that enough to say that the GPL no longer applies?
Sure, simple aggregates aren't counted as derivative works under the GPL, nor are scripts that don't violate the component programs. I'm pretty sure this isn't a simple aggregate. Several of gzip's functions can be found within the executable portions of the installer (NOT the data section), and those functions are called by code written by the installer's authors which, as far as I can tell, is reachable from main.