The keyword appears to be a specific promotional or tracking string associated with the high-octane Japanese live-action epic, Kingdom 4: Return of the Great General .
Link Kingdom 4 brings back the legendary Great General, thought lost after the Siege of the Three Bridges. Players command factions across a unified map, combining card-based unit linking (the “GM21” system) with real-time tactical battles. gm21linkkingdom4returnofthegreatgeneral exclusive