FSF GNAT (from your distribution) can be freely used. The GNAT that makes output GPL is the GPL GNAT edition downloaded from AdaCore's website.

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.

GCC only started to matter after UNIX vendors stop offering their SDKs for free actually.

Uneven C++ support was a big issue, too.