Hard to say, as yet. I'd expect it to be pretty open in the end, though; they're using LLVM and the Objective C runtime, both of which are open and usable on other platforms. I guess we'll see if they release the compiler.
Object C was created through changing the GCC and hence had to be released as open source due to the licensing of GCC (I believe) [1]. Swift is an apple invention and I would predict that they have no intention of releasing source code of it for availability on other platforms. Apple is a very closed ecosystem.
You are correct that the initial NextStep implementations of Objective C extended GCC.
However, GCC is not longer used by the Apple toolchain. They could easily close up their work on Clang and LLVM, and haven't. Further, they're obviously still contributing - they've recently added their ARM64 backend to LLVM.
The Apple software ecosystem isn't particularly closed. Much list iOS and OS X, they're based on a pretty open foundation, with proprietary libraries on top.
