Is The Legacy the same as The Gym?

No. Legacy is the story career. The Gym is account-wide recruits and cosmetics.

Does Legacy help me in ranked?

Yes, if you use it to learn one fighter and you still drill deny in Practice Mode.

You can. Most people who skip it also skip takedown defense. Do not be those people.

Offline career. Learn the fighter without a ranked timer in your ear.

What The Legacy is in UFC 6: the offline career from small halls to UFC arenas, and how to use it to learn a fighter.