Note that the quoted man gcc output is likely from a pre-release (10.5) version of the developer tools and the person submitting it is likely under NDA to NOT distribute/reveal that info.
The current (2.4) shipping public man information says the following:
-fobjc-gc
Enable garbage collection (GC) for Objective-C objects. The
resulting binary requires additional runtime support which is not
present in any released version of Mac OS X.
by private — Oct 20
The current (2.4) shipping public man information says the following:
-fobjc-gc
Enable garbage collection (GC) for Objective-C objects. The
resulting binary requires additional runtime support which is not
present in any released version of Mac OS X.