Undubs are not “ROMs” themselves but patches applied to a legally obtained copy of the game (a ROM dump from a user’s own cartridge). They require technical steps: extracting the game image, replacing audio files with Japanese counterparts (ripped from the original Japanese release), and repackaging the data.
If you’re diving into the ROM for the first time, expect a unique spin on the Suikoden lore: Applying Cheats to Suikoden Tierkreis Undub on DrDrastic suikoden tierkreis undub rom
Search for the on community forums like GBAtemp or ROMhacking.net . Undubs are not “ROMs” themselves but patches applied