I am trying to understand the Battle Gyms tab on the resource sheet and I am confused. I thought it was best to use double XP / gold smoothies on final boss and gold hoplins levels, but that sheet says it's better to use those smoothies on battle gyms.

It says "You only need one 2x gold or EXP smoothie for a run with a battle gym; simply Retry at the ranking screen and you'll retain the buff until you leave. Exercises cap at 3999 exp (300 for a full rep for 13.3 reps/exercise), while monster exp isn't seemingly capped."

What does this mean? Is it some sort of glitch?

As for gold, I don't see how the battle gyms are better too. Someone explain please.