On Linux, Maya is a shell script that sets up the environment to run, it explicitly checks for -d and then runs the executable maya.bin under the given debugger. Whether that is gdb, ddd, valgrind, idb or whatever is given.
On Mac, Maya is the actual binary. To run under gdb, its the other way round 'gdb Maya.app/Contents/MacOS/Maya'. But Xcode should jsut work fine, but as with Xcode 5, Apple do not ships gdb, you need ot use ~/.lldbinit (and settings set target.inline-breakpoint-strategy always)
