Romanji keymap are supported by third-party IME or ibus-hangul(libhangul). It is not supported by the MS Hangul IME.
The MS Hangul IME key sequence for 'G-A-N-A-D-A' is 'R-K-S-K-E-K'.
I've attached a patch to fix it. It passed the test on Windows10 22H2.
[use_rkskek_for_ganada.diff](/uploads/be1063e6a2d3e2793c61f9d3115a1e92/use_rkskek_for_ganada.diff)