Comment by sapphyrus

WOW64 syscalls are indeed implemented using heaven's gate, the 32-bit ntdll calls into a "wow64cpu.dll" module, which does the long-mode transition and ends up calling into the 64-bit ntdll. Unfortunately manual syscalls are still possible (and widely used) on windows, either by hardcoding syscall IDs for common versions or performing very rudimentary "disassembling" of the ntdll syscall stubs.