I haved the same issue time ago.
If you have the disc release of the game, you need also to own all the previous dlc (requiem pack and summer bonus episode). After that, you can purchase the GOTY upgrade from the in-game shop, because it seems the only way to obtain it. Better to use the game’s shop for any hitman related purchase.
Btw, keep in mind there is not additional download: all the goty contents it’s already present in the game’s source after the last patch, but it’s locked until you pay for those. I confess i’m a bit upset for that.