Yep, the GNAT toolchain in Debian is actively maintained and they even build cross compilers for all of Debian's supported architectures. Combined with a Linux RT kernel, you can get some pretty good performance with only GPL code.
If you need to build for bare metal rather than Linux, things get a bit more complicated as GNAT requires some runtime code to setup things like systick and interrupts. You can use AdaCore's non-GPL runtime for non-commercial purposes, cortex-gnat-rts on top of FreeRTOS, or RTEMS, which natively supports Ada (though it's poorly documented).
I've also had some luck using crosstool-ng to build gcc toolchains with Ada support, but you'll still need to find or write a runtime library for bare metal.
