Hi,
The support for a specific language standard (e.g. C++20) comes from the gcc version included in your toolchain. VisualGDB works just fine with it, as long as your toolchain supports it.
The wizard indeed shows language standards up to C++17, but you can always change the standard to C++20 via project properties, once you have selected a toolchain compatible with it.
For CMake projects, you can also just set the language standard directly by editing the CMakeLists.txt file. VisualGDB will automatically pick it up once you reload the project.