The "gm21link" refers to the specific integration patch that bridges the mobile and desktop versions of the game, ensuring that the Return of the Great General event is seamless across platforms. Exclusive to this link are:
GM21LinkKingdom4ReturnOfTheGreatGeneral: Exclusive Deep Dive into the Return of a Legend gm21linkkingdom4returnofthegreatgeneral exclusive
To ensure you are running the correct version, check your settings menu for the build number. If you are using the exclusive link, your inbox should contain a "Veteran’s Welcome" package, including 500 gems and a 3-day recruitment speed-up. The "gm21link" refers to the specific integration patch