I already own a license for Immerse Gaming software. Do I need to buy a separate license for Immerse Gamepack app FFXIV?

Yes, you will have to buy a separate license for Immerse Gamepack app FFXIV. Immerse Gaming software works on the output sound of the game (Ex: 5.1 or 7.1 output), whereas Immerse Gamepack FFXIV works inside the game engine at the sound object level. Also, Immerse Gamepack app works only with Final Fantasy XIV edition games, and you can pick any headset you have, whereas other Immerse Gaming products are compatible with listed headsets and their model only, and it works well in any game you play with the supported headsets.