I agree it does have a macro facility. I wasn't trying to say otherwise. Actually from the readme
-- The Gambit-C system conforms to the R4RS and IEEE Scheme standards. -- and -- - unhygienic macros --
These two together suggest that it doesn't. Not that it really needs it as long as it has defmacro-style macros. I suspect that if you really really wanted syntax-case and define-syntax you could write them with a defmacro style macro.
On a side note, I had nearly a million threads running last night, on a stock pc, before I started running into timeout issues. Gambit 4 is an awesome system.