Yes, the proposal is tailored so that other than simple syntax support no new semantics need to be implemented within GCC to support defer, though clang will need to finally add support for nested functions--in spirit if not the literal GCC extension.[1] The proposal also gives consideration to MSVC's try/finally to minimize the amount of effort required there to support defer.
[1] Because defer takes a block, not a simple statement. And deferred blocks can be defined recursively--i.e. defer within a defer block.
>the proposal is tailored so that other than simple syntax support no new semantics need to be implemented within GCC
Not just GCC, but you're right it's tailored, to the same "unwinding" queue that C++ destructor, stack-VLA de-allocation and __attribute__((cleanup)) shared, won't fit into the current state of language otherwise.
Clang share more frontend between C and C++ so I imagine they can implement it as hidden C++ lambda scope-guards, the nested scenario is just full-capturing lambdas inside another.
[1] Because defer takes a block, not a simple statement. And deferred blocks can be defined recursively--i.e. defer within a defer block.