You do not need to take any special action with any operating system to get it to support the GPL. What Apple have done is done special action (lots of EULA/contracts/etc.) that explicitly disallow the GPL.
The reason (I think) Apple are explicitly disallowing the GPL is because it would be incompatible with the DRM type systems that Apple use. It's more a case of 'If Apple distribute GPL software on their App Store, they might have to give away the DRM keys and allow anyone to run any programme on their iPhone'. That situation is something Apple don't want.
GPL programmes are still allowed on OSX obviously, and I think some of Microsoft's mobile and/or app store thingie explicitly ban GPL as well.
Show me the places where Apple EULA "explicitly" disallows the GPL (and not just includes things that are incompatible with the GPL). There certainly is GPLed Software in the app store, even with a notice included (see Battle of Wesnoth). If it was explicitly forbidden, that would not happen.
My interpretation is that Apple just doesn't care. The want Apple-signed code on their devices and if your license doesn't allow signed code, they won't help you. This by itself is sad and I would very much hope for that to be different, but not "explicitly disallowing".
Apple do not explicitly mention it, however they have chosen to add extra restrictions to their licence, which goes above and beyond a normal "Download software from this web host". Explicit clauses that make the GPL incompatible are the same as "explicitly banning GPL apps".
The GPL is a massively popular licence, I believe Apple would have considered it. They appear to have rejected it.