This is something managed by the ESP-IDF itself rather than VisualGDB. Please check with Espressif whether the ESP-IDF version you are using supports C++17 and whether there is a configuration option that enables it.
Good to know it works. We are also planning to release a fully integrated version of this toolchain (including ESP-IDF 4.0) together with the upcoming VisualGDB 5.5 Preview 1 (the internal structure of ESP-IDF 4.0 has changed compared to 3.x, so we are updating VisualGDB to accommodate those changes).