The DMG for the Mac version of Duke 3D doesn't appear to include the source or a Harvester manual, though it seems to be based on Boxer, not just DosBox. The Mac PKG for Ultima does have the source (dosbox_svn.zip), but no (edit: out of place) manuals as far as I can tell.
I wonder if the Harvester manual you're seeing is a GoG artifact from reusing the same DosBox setup as Harvester and they forgot to remove it, as the source from Ultima includes a file called "harvester_mouse_clean.diff".
I found and downloaded Jill of the Jungle, which is a free game that uses dosbox and is available on mac.
after you install the game, check out
<dir>/Jill of the Jungle The Complete Trilogy.app/Extras/Manual.pdf
E.g. When you use a patched version of Linux in your router, then you'd need to make those patches / the patched code available, but you would not need to make the code of your web GUI available.
If you are using libviralgpl.dll in your ProprietarySecrets.dll though, your entire application becomes a subject of the GPL.
In general, a reasonable boundary is that if it runs the GPL'd work as a command, then it's not a derivative work, but if it links against the GPL'd work, then it is. (This is of course not a distinction established in law, which knows nothing of subprocesses and linkers, but it's a good approximation, most of the time. Making a modification to a GPL'd binary to expose a particular interface for another non-GPL'd process to call probably makes that non-GPL'd process a derivative work, though. Conversely, using a standardized API that happens to be implemented by a GPL'd library probably does not make the program linking it a derivative work.)
The GPLv3 says:
> A compilation of a covered work with other separate and independent works, which are not by their nature extensions of the covered work, and which are not combined with it such as to form a larger program, in or on a volume of a storage or distribution medium, is called an “aggregate” if the compilation and its resulting copyright are not used to limit the access or legal rights of the compilation's users beyond what the individual works permit. Inclusion of a covered work in an aggregate does not cause this License to apply to the other parts of the aggregate.
GOG presumably believes that they are shipping an aggregate, and DOSBox is an individual work, and the game they're shipping is a separate individual work, and the wrapper script to put them together is its own work, independent of the specific game or specific emulator. They must comply with the license of DOSBox, i.e., they must provide the corresponding sources, but that license doesn't apply to other things in the aggregate.
For comparison, the Linux kernel also says:
> NOTE! This copyright does not cover user programs that use kernel services by normal system calls - this is merely considered normal use of the kernel, and does not fall under the heading of "derived work".
...and strangely - I don't know if this is only the mac version - they randomly include the manual for a PC game called Harvester