1. Have MSVC compiler and CDB debugger installed.
2. Have a Qt version built with MinGW, a matching MinGW compiler and gdb debugger installed.
   I used Qt 6.3.1 and MinGW from Qt's online installer.
3. Use that Qt version to build a qmake based project.
   I built a simple console project from the command line.
4. Have gcc, g++ and gdb from step 2 in PATH variable. Do not have qmake in PATH.
5. From this environment, start Creator with empty settings., e.g. using the parameter -tcs
   Creator can't find Qt yet because it is not in PATH.
6. Open the project you built in step 3 in Creator.
7. In the "Configure Project" view, import the build from step 3 and click "Configure Project".
8. Go to "Edit" -> "Preferences..." -> "Kits" -> "Kits".
   There will be an automatically created 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.