Ummm... let me just say "read the paper, please" :-). I'm fully aware that compiling the same source with different compilers will (normally) produce different executables.
> Or are you suggesting that A-G and B-G then be used to again compile Gcc, and *those* binaries be compared? That would tell you that either A and B were not subverted, or were subverted in exactly the same way...
That's the basic idea, sort of. Given certain preconditions, you can even recreate the original executable with a different starting compiler.