This is absurd.
Kakashi CANNOT match Gai's speed with 6 Gates. It's absolutely stupid to think that.
Gai is a taijutsu expert/specialist. This involves using his physical body including speed, strength, agility, etc. He beats Kakashi in anything related to taijutsu and it's attributes. Let alone with 6 Gates...
It's just like the OP said, Gai and Kakashi were up against 6 Bijuu. Why would Gai zip towards them at 100% by himself? The plan was to use their full strength, but work together, which is why Gai is slowing himself down to match Kakashi's speed. So they can stick together and fight in unison...Not have one of them barge in by themselves and get themselves killed 1v6...
Seriously, this isn't a hard concept to wrap your heads around. It's common sense.