gnat (gcc's Ada implementation) needs an Ada implementation to bootstrap (just like gcc needs a C++ compiler). If you have gnat[0] installed on the host, you also get a gnat for the targets.

(side note: The chromium-os coreboot-sdk package uses a binary toolchain from AdaCore for the bootstrap, so within cros_sdk the cross compilers are all Ada-aware.)

But yes, it's not exactly obvious.

Patrick

[0] conditions apply: it needs to be the same build as gcc so that they can interact (some distros mess this up) and it must be sufficiently new (although we integrated a bunch of hacks to support older compilers than usual)