I've been developing an Ada runtime system based on FreeRTOS, which has worked very well.
Most of the code is either based on FSF GCC, or else directly copied, and uses GPL version 3 with the GCC Runtime Library Exception whose effect is very similar to Clause 1 of the FreeRTOS GPL exception.
I see that the FreeRTOS is licensed under GPL v2, and not (as I've seen elsewhere) under "either version 2, or (at your option) any later version".
OPENRTOS wouldn't have this problem, of course, but is there any chance that you might consider upgrading your FreeRTOS licence terms to v3, or at least include the "or later" wording? because there is no legal way to combine code under [plain] GPLv2 with code under GPLv3 in a single program.
How is your run-time going to be distributed? Unless you are
distributing source code then you can distribute "under the terms of
your choice" - as per the exception text.
(sorry for delay - I expected to be auto-subscribed to the thread)
The trouble is that a GCC Ada RTS has to be distributed as source, because user code has to be compiled against it.