Using a compiler whose code is under GPL doesn't make your code GPL. But using the GPL runtime library might be an issue. The FSF GNAT version has a runtime library exception which allows you to use the runtime library in non GPL code. The Adacore version has no such exception (at least it didn't when I checked last time).
But breaking GNAT's license doesn't make the software created GPL
If you do not comply with the terms of the GPL license you're no longer allowed to use the code/software licensed under GPL. Under GPL 2, you can continue to use the code/software after the license violation is removed. Under GPL 3 the copyright holder might terminate your license (see section 8 of GPL 3).
Even then there can be a question of whether your code is derivative of the runtime, or of the standard. If the latter, then you might escape the licensing quagmire
I don't think you can use Ada without dependency (i.e. making a combined work) of a runtime (maybe someone knows a good way without sacrificing relevant Ada features, I don't).
The basis of derivative work, even when RMS was bullying projects with it, was whether your code was dependant on the GPL implementation, or say, could just as well be built with another vendor's library - even if that one was GPLed too, so long as it wasn't derivative of the first runtime.
Essentially a case of "coding for standard interface offered from multiple sources where the interface itself isn't GPL" vs "Your code requires rework in order to work without this specific GPLed dependency".
This is all grossly simplified, but among other things, it means that separating GPL code to another binary you talk over any form of IPC doesn't necessarily mean you're avoiding GPL.
whether your code was dependant on the GPL implementation
It's rather "whether your work", not only your code. GNAT generates runtime library dependencies even if you are not aware of it. So the result is a "combined work", because the runtime library is linked to your code (otherwise your application wouldn't work).
Comments
Using a compiler whose code is under GPL doesn't make your code GPL. But using the GPL runtime library might be an issue. The FSF GNAT version has a runtime library exception which allows you to use the runtime library in non GPL code. The Adacore version has no such exception (at least it didn't when I checked last time).
If you do not comply with the terms of the GPL license you're no longer allowed to use the code/software licensed under GPL. Under GPL 2, you can continue to use the code/software after the license violation is removed. Under GPL 3 the copyright holder might terminate your license (see section 8 of GPL 3).
Meh, the default behavior of GNAT is to bundle the runtime in the executable anyway...
Even then there can be a question of whether your code is derivative of the runtime, or of the standard. If the latter, then you might escape the licensing quagmire
I don't think you can use Ada without dependency (i.e. making a combined work) of a runtime (maybe someone knows a good way without sacrificing relevant Ada features, I don't).
The basis of derivative work, even when RMS was bullying projects with it, was whether your code was dependant on the GPL implementation, or say, could just as well be built with another vendor's library - even if that one was GPLed too, so long as it wasn't derivative of the first runtime.
Essentially a case of "coding for standard interface offered from multiple sources where the interface itself isn't GPL" vs "Your code requires rework in order to work without this specific GPLed dependency".
This is all grossly simplified, but among other things, it means that separating GPL code to another binary you talk over any form of IPC doesn't necessarily mean you're avoiding GPL.
It's rather "whether your work", not only your code. GNAT generates runtime library dependencies even if you are not aware of it. So the result is a "combined work", because the runtime library is linked to your code (otherwise your application wouldn't work).
See e.g. https://www.gnu.org/licenses/gpl-faq.html#GPLStaticVsDynamic
Your source code may not be a derivative of the runtime, but your object code clearly is.