You would still want to retain the same tire diameter overall. Lots of us run 29x3.0 in place of 26x4.8 or 27.5x 2.8 for a bike built for 26x4.0. Running smaller diameter wheels will drop your Bottom bracket and result in pedal strikes.
With fatbikes tires matter. With a set of tubeless compatable rims and a pair Jumbo Jims and it will feel like a roadbike compared to what the gravity monster comes with.