set sysroot $(NDK)/platforms/$(#NDK.target)/arch-arm
or any other command.
Where $(NDK) - system's variable and $(#NDK.xxx) - C::B's global variable. It is perfectly works everywhere in project's properties except "Additional GDB commands".
Then, from GDB console, i'm calling "show sysroot" and it prints "$(NDK)/platforms/$(#NDK.target)/arch-arm" without any changes.