its possible to beat the game without doing them,but its really hard,i suggest just going thru the game again so yuo can build up your chars some more.
(btw,when you do beat the last boss save your game and load it back up to do another quest thats only open till you beat the last boss) and talk to captain matthews to go back to the last dungeon to beat the last boss again and again and again so you can get a lot of exp for the quest to come
Yeah the GS campaigns you don’t have to do them but it be better if you did. Plus you get some really cool double attacks from them!