How do you unlock the final story in sonic FGX?

It seems you're asking about a game called "Sonic FGX", but that doesn't appear to be a real Sonic game title.

Are you perhaps thinking of one of these?

* Sonic Frontiers: This is a recent 3D open-zone Sonic game.

* Sonic Forces: A 3D game with both 2D and 3D levels.

* Sonic Generations: A game celebrating Sonic's history with classic and modern gameplay styles.

To help me figure out the game you're referring to, please provide more information:

* Platform: What system are you playing on (e.g., PC, PlayStation, Xbox)?

* Year of release: When was the game released?

* Any other details: Is there a specific stage or character you're trying to unlock?

Once you provide more information, I can help you figure out how to unlock the final story in the game you're looking for.