Two different jobs
Fitbod answers "what should I do today?" by generating the session. GYM RPG answers "why would I do it?" by attaching XP, skill points and a party boss to the sessions you log.
Price
Fitbod's paid plans are in the low-to-mid teens per month on the US App Store. GYM RPG Pro is $7.99 a month and $49.99 a year, and GYM RPG has a usable free tier.
Can you use both?
Yes. Some people use a planner for the programming and a tracker for the habit side. Log the planned session in GYM RPG for XP and boss turns.