1. Have MSVC compiler and CDB debugger installed.
2. Have a Qt version built with MinGW and a matching MinGW compiler in PATH variable.
3. Start Creator with empty settings.
   E.g. using the parameter -settingspath <empty directory>
4. Go to "Tools" -> "Options..." -> "Kits" -> "Kits".
   There will be an auto-detected Kit using the MinGW Qt version. For this Kit, the CDB debugger is set. Creator even shows a warning that these don't match.

gdb was automatically detected, it was just not selected for the kit. Since Creator already noticed that cdb does not match, it should be able to select gdb instead.