I think Luffy still has Gear 6 to pull out. And that will be needed against Imu and Blackbeard
Gear 1 - All the opponents up until Alabasta
Gear 2 & 3 - Lucci pre time skip, Pacifista
Gear 4 - Doflamingo (Crocodile retcon, Moriah without Nightmare Luffy, Enel without Fruit hax)
Gear 5 - Kaido (Big Mom, Akainu)
Gear 6 - Blackbeard and Imu
So yes, BB and Imu should be above Gear 5 forcing Luffy to go into the hidden 6th Gear.