gdb from Path is not being autodetected

1. Have g++.exe and gdb.exe in your PATH environment variable.
2. Start Creator with empty settings.
3. Check the autodetected toolchains in "Tools" -> "Options..." -> "Build & Run" -> "Compilers".
   g++ from PATH is available.
4. Check the debuggers in "Tools" -> "Options..." -> "Build & Run" -> "Debuggers".
   gdb from PATH is not available.

Because of this, automatically added kits don't have a compiler, see QTCREATORBUG-12186. Since Creator can detect g++ from PATH, it should also be able to detect gdb.