There are several Final Fantasy gachas. All the Bravest is absolute horseshit, never touch that. Final Fantasy Record Keeper looks okay, but it's mostly a nostalgiatrip through the old boss fights and grindy as fuck in addition. Plus, it doesn't even work as a nostalgiatrip if you hadn't played the games or just played them once and so don't really remember.
Final Fantasy Brave Exvius is at least okay in the beginning, since stages cost no stamina before account level 10 and you get a huge stamina bank after that. I honestly didn't play far so I can't say how much of a scam the gacha turns out to be in the long run, but at least the first few hours can be a good distraction.
I've not tried out Mobius Final Fantasy, so no opinion there.