GDB + softdevice ?

[ypearson]

Can you use GDB for your app code if soft device was merged in the final hex?

[Wojtek]

If by GDB you mean compiling on GCC, it does not interfere with softdevice. You just separately flash app compiled in GCC and funcions calling to softdevice communicate with it by SVCALLs. Just keep memory layout as specified in softdevice specification.