INDIGO
The question is not about loss of FMGC. It is about use of FMS in APP mode for the precapture of localiser. You may have FMGC but if GPS primary is not available the enhanced loc capture is not available. But you still have FD/AP. You don't need to fly raw data. Similarly the Loc Convergence Function requires NAV mode. You can be flying in HDG mode with FD or with AP on but this function will not be available.