>It's all very confusing to me because I thought Apple's assembler
>*was* gas.

Sort of - if I understand it correctly, it is set of separately
maintained patches (for Mach-O) to an old version of gas. It is most
conveniently thought of as a different assembler. Not that it matters,
as the syntax and pseudo-ops supported by gas itself vary between
platforms, as the aim is to behave as the native assembler.

If you care, you could look at the cctools sources - they are publically
available.